SPOILER ANSWER! DON'T CLICK IF AVN'T SEEN MOVIE

Maybe becoming evil -was- his reaction to his troubled past. He did come across as rather evil, especially after ditching them at the incinerator...

I dont blame him for being evil. He was left behind. I mean, yeah it was kinda mean to take it out on everybody. But seriously?!?! I dont blame him! Otherwise, He's the sweetest, cutest thing!

Now don't get me wrong, I loved Lotso a lot. But yes, I'm afraid I have to say that I did find him evil.

He had his reasons though. Being replaced by his old owner really messed him up. He started out being pretty much exactly like Woody. He was completely dedicated to his kid. When he, Big Baby and Chuckles got lost, he was the one who wouldn't give up on getting back to her, much like Woody would never give up on getting back to Andy.

But after going through a long journey to get back to her, seeing that Daisy had simply bought a new bear destroyed him. It made him believe that he had never been unique and special to her. From that moment on he believed that the life of a toy was meaningless. In the end no kid ever truly loves you. If they lose you they can just replace you, and once they’re bored with you they can just throw you away.

This realization made him became bitter and twisted. He stopped being nice and took out his anger on other toys. He especially hated any toy that still had love for their owners. He saw them as fools. He probably saw much of his old self in them.

He was a very complex villain, which is why I loved him so much. I felt very sorry for him.