Guy Gardner and the Dark Lantern [Theory]
There's something about the Lanterns series that everyone is talking about, but I think people are getting it wrong or overlooking a major detail.
Why did Guy experience that sudden personality shift, and why didn't he confront John Stewart at the end?
1. Tom King's fundamental misunderstanding of the character
Tom King holds Guy Gardner in very low regard and constantly uses him as pathetic comic relief in his stories simply because he doesn't understand him. Guy is essentially a hero who knows no fear, but also has zero filter to "be polite."
TL;DR: Tom King is a lazy and bad writer.
2. A coherent explanation for Guy's sudden fear/personality shift: The Dark Lantern
Within the show's canon, Guy doesn't know what fear is and has never truly faced it as entity. He knows little about Qward or Sinestro; he’s basically been busy doing PR damage control over the Manhunters' massacre that Hal Jordan and John Stewart were involved in. Even though he has faced other threats, he has never been directly exposed to the emotion/power of fear itself.
In the show's canon, the Dark Lantern is activated by fear. When Guy arrives to talk to John, he drops his guard, completely unaware that the Lantern might be corrupting the area with fear—or even that his own ring might be harboring the Yellow Impurity. Exposed to a cosmic force he has never encountered before, the power of fear infects his mind. This causes Stewart's words to hit him with a psychological impact that Guy would normally never succumb to (whether in show canon, the DCU, or the comics).
3. All about a Guy
I'll leave you with this: Guy Gardner is my favorite Green Lantern. Not just because he's fun to read, fearless, and unfiltered, but because he's genuinely a good person. Guy is a good guy.
I hate when writers use him for cheap jokes.
I first fell in love with his character during his peak era under Beau Smith and Mitch Byrd (in Guy Gardner: Warrior), where he faced off against Hal Jordan / Parallax without even wearing a Green Lantern ring—using Qwardian tech / Sinestro's yellow ring—and nearly beat him! Out of the entire Corps, only he and Sinestro came close to stopping Parallax during Emerald Twilight. Hal destroyed the entire Green Lantern Corps, but couldn't destroy Guy!
Guy didn't trigger the destruction of Coast City or go mad (Hal Jordan), he wasn't possessed by an evil entity like Parallax, he didn't fail to save Xanshi from destruction (John Stewart), nor did he blow up Mogo or make world-shattering mistakes (Kyle Rayner). There's a strong case to be made that, as loud as he gets, Guy has made the fewest catastrophic mistakes out of all four main Earth Green Lanterns.
If you want to see why he's such a top-tier hero, read Peter J. Tomasi and Patrick Gleason's run n Green Lantern Corps.