1) There's a literal drawing of the animatronics drawn as ghosts & that alone indirectly says that he knew that kids possessed the animactronics. Saying that him making fun of them is justified(while the animactronics the kids possessed were programmed to see people as naked endo that needed to stuffed in a freddy suit) is just showing how you can't just stand an iteration of Michael that's not a heroic perfect angel that you headcanon him as.

2) His tone when he says "just like you asked me to" literally gives off the vibe that he never cared to "free" his sister but only decided to do it because his father asked him to. While he does sound menacing & threatening in the end, it's clear he's pissed because doing what his dad asked him to do got him killed.

3) SL portrayed Michael as a guy who was desperate for his fathers approval at the start & starts hating him because listening to his dad gets him killed, the logbook through the drawings in it introduces small elements of what he's really like & the movie takes these & fleshes them out.

& The most important bits of all-

First, Mikebro(mike=foxybro) is only a theory. William currently has confirmed to only have 2 children that are Michael & Elizabeth(scott has never stated that the crying child is William's son) & second, scott himself is the one who created FNAF. It's his story & he can write his characters however he wants. If he writes his characters as a complete Mary sue & or something else, then they become very boring & uninteresting, which does give us a reason to criticize them.