When did Sid become evil?

Maybe it was because of the neglectant parents he had, and as a result he watched a movie or a TV show he shouldn’t have at a very young age (like, 3 or younger) and it messed him up?

I mean keep in mind the kid isn’t evil, he doesn’t know these toys are alive, but he does seem like a very badly behaved child over all.

There are a myriad of reasons for Sid’s behavior. I touched on them slightly in the fic Belonging. I surmised that Sid’s parents gave Hannah Sid’s old baby toys and that began his sibling rivalry and his penchant for destroying toys -Omar