Theories on fnaf characters and thoughts/speculations

Why Michael Bullied Chris/CC

Step Closer definitely offers a pretty similar case if any of you haven't read it and don't want to be spoiled well... Major Spoilers

Pete and Chuck are pretty-similar to Michael and Chris and comparing certain parts of the story it really ties these two.

Pete being the older brother to Chuck and his parents having no time/working all the time. Pete is expected to take care and watch over Chuck, HE didn't get say a word about it.  He couldn't hang out with friends, couldn't really play sports he had to babysit Chuck, to make it worse nothing mattered about Pete it was all about Chuck. This eventually had him drag Chuck to go see Foxy.

Now comparing this to Michael its not that different Michael takes any chance to jump out at CC, locking him in his room and the parts room. To the point of the big birthday surprise as he and his friends all laugh at him until he goes limp in Fredbear.
Both these characters hit a breaking point Pete's was from getting cursed from Foxy, Michael's is from seeing the bite... both were too late...

Logically we would be pretty upset too if we had to watch over a younger sibling because our parents are so busy they don't have time for us, TO TOP it off we Don't get a say in it.

So the bullying/tormenting behavior stems mostly from family issues... as well as likely feeling powerless and Michael's desire for control.
William is technically seen working in Fredbear's likely all the way to night so he basically is absent to these two.
The huge responsibility of having to watch over a younger sibling and not get to have a say in it, yea I don't blame Michael for getting mad at it. Michael is pretty much using CC as a way to release his anger and frustration on an outlet really.