Look I have a theory, I think, sorry I KNOW that I have just solved the lore, Michael Afton is a time traveler, yes, let me explain to you because I have a lot of evidence to say that.
First of all we know that Michael is too similar to William, he literally said "they thought I was you", this means one of two things, Baby is stupid or most probably, Michael looks exactly like his dad but younger, you may think "mmm yeah he's his son" no no no, I have a better explanation for this, they are the same person, Michael Afton is William but from the future, it's obvious, he's from a timeline where he didn't kill anyone but maybe instead he did something that he regrets like IDK whatever, and I know, where does he get a time machine?, I have an answer for that too, Into the Pit, the ball pit is the prototype of the time machine William was working on (prototype because the "time travel" in Into the Pit is different so it could destroy my fcking theory), he used it but he's stupid so it failed and instead of arriving at a certain point of the timeline with his current age, he arrived at that point of the timeline being a baby, that's stupid?, I have actual evidence for this, you remember The Immortal and the Restless?, if we assume that the baby is Michael, the vampire William and Clara Mrs Afton everything is obvious, "the baby isn't mine" that's actually true, cause Michael wasn't born, he just appeared in the past because he is William but as a baby,and here's the last part of this completely confirmed part of the lore, why is William evil?, Michael is the cause, he fucked William's life when he just appeared in his life, that's why William is evil because he's angry and he's a bad father too, that's why he started killing children because he hates children (just ignore the fact that he literally has 2 maybe 3 more children) because he hates Michael.
I have something else, you remember the "mike killed all"?, yeah, it was real, Michael actually killed all, not directly but he indirectly is responsible for the death of all William's victims, because he's stupid.
Yes Scott actually traveled to the late 19th century and wrote the London Bridge song with the Mike kill all subliminal message.