u/SpeedyPuzzlement

Even with only the information available to him, I don't understand how he thought it was a good idea.

1. By the time Renly flees the capital, it’s clear that the Starks will war with the Lannisters. There’s a reason why three-way wars are relatively rare - the first two parties to make an alliance will boost their odds of winning the whole thing, and there is zero chance the Starks or Renly will accept a Lannister alliance. Renly and the Tyrells are far more suited to winning the peace in a Stark alliance rather than going their own way. Demanding to be recognized as king is an unnecessary barrier to getting the Stark armies ASAP.
2. Adding more kings to the war will prolong the fighting and destabilize Westeros, especially since he didn't know whether Stannis would declare. This invites foreign intervention (sellswords, Iron Bank, Daenerys, White Walkers). It also encourages other regions to pursue their own agendas (Iron Islands, Dorne). Even if Renly got the throne, self-crowning would haunt him in the long term.
3. As the Master of Laws, Renly had three ironclad reasons to call the banners of the Stormlands and the Reach: putting Jaime on trial for attacking Ned in King’s Landing, justice for Ned’s execution without a trial (precedent with Aerys), and stopping Gregor’s burning of the Riverlands (Ned’s order as Hand of the King). All he had to do was say “I’m doing my duty” and the optics would’ve been perfect. He had three legitimate and mutually compatible casus belli. He could have also accused the Lannisters of assassinating Robert, seizing the Stark daughters, and incest. Instead, he threw away duty, law, and precedent for an obvious power grab that any future usurper could take inspiration from.
4. It’s clear that the Lannisters will go after Stannis. If Stannis falls in battle, Renly becomes the legal successor (regent to Shireen at worst), gets another casus belli (“avenging my brother”), and saves a LOT of heavy lifting in terms of waging war. Whereas self-crowning alienated Stannis for predictable reasons.
5. Even if Stannis defeats the Lannisters, he is likely to alienate his subjects (renouncing the seven, burning people alive like Aerys, reputation of no mercy as shown by Davos' fingers, banning brothels). Nothing would support Renly's rationale of being a better king more than letting Stannis fumble the throne while he serves as Hand or, at worst, as Lord Paramount. Remember when people compared Tywin and Aerys? Again, Renly and the Tyrells could easily win the peace. They are in prime position to manufacture some crises (food shipments to King's Landing), scapegoat Stannis, and encourage his downfall by smallfolk riot, Great Council, or Duskendale 2.0. I'm surprised Olenna didn't decide that a secret pact and a Purple Coronation would've been miles cheaper than the time / money spent battling Stannis.
6. Let's not forget about worst-case scenarios. If you ally with Robb and betray him later, he'll take your head. If you ally with Stannis and betray him later, he'll take your head or burn you. But if Joffrey / Cersei / Tywin end up on top, you and your loved ones will beg for death. Everyone knows the Rains of Castamere. Everyone knows what happened to Elia. Joffrey just executed the father of his betrothed! Even if we accept that self-crowning increases the odds of taking the throne, is it worth the friction with Robb / Stannis and the increased risk of ending up in a dungeon with Amory Lorch and Gregor Clegane? I don't think so.

In summary, Renly should never have crowned himself. He could've had the Reach, the North, the Riverlands, and Dragonstone doomstacking towards King's Landing, with the Iron Islands and Dorne gladly joining the winning side. Instead, hubris led to his downfall.

For me, much of the appeal of the Souls series is that it's hard but fair - you've probably heard that a lot. But what does that actually mean? Spoilers for Dark Souls.

Understanding Fairness

I will use some quotes from Hidetaka Miyazaki, the game director.

https://makegames.tumblr.com/post/4949412103/firm-but-fair
"Any player can clear any obstacle simply by learning from mistakes and paying close attention (#1). The reasons for failure must always be clear and understandable (#2). Every problem must have multiple solutions, so that the player can tackle it in whichever way they want (#3). The game’s controls can never be a factor from which difficulty is derived (#4). There must be the possibility for miracles to happen - those magical moments that spread stories outside of the confines of the game world (#5).

So long as an obstacle passes those five criteria, we are happy that we have achieved the maximum level of difficulty, while retaining the necessary element of fairness.”

I like this rubric a lot. Understanding why you failed (#2) goes hand in hand with paying attention / learning (#1). Fairness isn't optional: it's necessary for the Souls formula. Note that Miyazaki speaks of players and situations, not in-game characters. He's not talking about how it's "unfair" that the Chosen Undead can't dash like Ornstein or inflict Curse like Seath.

http://soulslore.wikidot.com/das1-game-no-shokutaku
"Murohashi: Why don't you get toxic from looting it [the Dung Pie]?
Miyazaki: That wouldn't be fair gameplay."

"Isomura: Eingyi … was made to trick players into attacking them, right?"
"Miyazaki: Well, the TALK option should have been showing a bit earlier."

"Isomura: About Mr. Solaire. Isn't it impossible to finish his story if you play normally?
Miyazaki: It's impossible to do it without hints. For Solaire, there's a shortage of hints in general … in this case (and in others), the conveying of hints is a subject we need to work on."

Miyazaki tightens this definition to describe "gotcha" moments - being penalized in a way that you couldn't have reasonably predicted - to be unfair. The toxic dung pie scenario can easily be avoided by learning from mistakes (#1) and is realistic (picking up fecal matter would be harmful to your health), but you can't tell it's a Dung Pie before you pick it up so it's unfair. 

https://www.pcgamer.com/games/rpg/elden-rings-developers-know-most-players-use-guides-but-still-try-to-cater-to-those-who-go-in-blind-if-they-cant-do-it-then-theres-some-room-for-improvement-on-our-behalf/
"We try to cater to the player who is completely blind and wants to go through organically. If they can't do it, then there's some room for improvement on our behalf …"

If failures should be foreseeable, this naturally begs the question of what players ought to be able to reasonably predict. Clearly there is a gradient of experience from "never played a video game before" to "just finished every other game in the Souls series". And everyone of course has their unique life experiences that might make them better at detecting various dangers - you might be able to spot that Patches is lying or you might look up more often. Sometimes you just get lucky and make a decision that avoids death. When Miyazaki speaks about the completely blind player, he refers not to a real player but to an ideal audience for which games should be designed.

http://soulslore.wikidot.com/das1-variety-is-the-spice-of-death
"... there are a few ways we prevent users from drifting away … We have not created a game where players who react faster or press buttons faster fare better than others … when a player dies, we try to leave a sense of 'maybe if I try a different strategy I can succeed'."

Miyazaki makes the definition of fairness even more strict: you should not only understand why you died but feel as though it could have been avoided. Furthermore, he does not want reaction times or clicking speed to be the difference between success and failure. I get the sense that Miyazaki wants his games to be accessible to everyone, regardless of background knowledge or intrinsic talent. And that's a beautiful ideal.

http://soulslore.wikidot.com/das1-variety-is-the-spice-of-death
"We don't want players to be frustrated by doing the same things over and over … We want to give players many options, even if that means they use the sword that 'fits best in the hand.'"

https://gamerant.com/dark-souls-3-incomplete/
"I never want it to be where you have to follow the rules completely, where you have to do things exactly as the designers intended."

If we circle back to the rubric (#3), it also seems that Miyazaki considers player freedom a crucial component to fairness. This does create a tension between difficulty and fairness: if you ramp up the difficulty too much, players may need to farm or change their build. Conversely, can we really call a game difficult if it allows the players to succeed no matter what they try? 

Fairness isn't binary. It's a metric that requires balancing freedom and difficulty. And when striking this balance, it's better to favor players. This is especially important when a strategy might be obvious to developers, but not to players. How many players used Dung Pies to avoid the blowdart snipers? Per Miyazaki, "There's actually a planned and odd way of passing through Blighttown … throw them [Dung Pies] to get toxic, then you can go on."

http://soulslore.wikidot.com/das1-variety-is-the-spice-of-death
"We are trying to create a game that is spicy and we want to make it as spicy as possible, but still edible and also to make it taste good that leaves you wanting more."

This also gives a new meaning to "hard but fair". I don't think it's correct to say that Dark Souls packs in as much difficulty as possible while not crossing some "fairness" line. Instead, the beauty of Dark Souls is that it manages to be as difficult (spicy) as it is while also being as fair (edible) as it is.

In Summary

Fairness is when:

• You can predict and avoid any failure by paying attention - no guides needed.
• Failure is educational, never poor reflexes or janky controls. It feels avoidable.
• Players have the freedom to choose their path to success (within reason).

Does Dark Souls always live up to this ideal? Of course not. But it is a beautiful design philosophy: the foundation of a successful genre and a useful benchmark for any game.