As far as I'm concerned, he was always a monster. Having him do one nice thing at the end doesn't really make up for the fact that he's essentially an allegory of a white slaver. He was exploiting the other toys at the daycare center and making it a living nightmare for everyone who wasn't sucking up to him. He was a truly despicable character after he went insane.
Then again, Sid wasn't redeemed (he was traumatized by the other toys), neither was Stinky Pete (given to a little girl who draws on her toys). In fact... are there any Pixar villains that redeemed themselves? I mean that Richard Kind voiced Grasshopper in Bug's Life is all I can think of at the moment. Maybe the guy who turned into a bear in Brave after he was put to rest. Heck, I hear that things were going to be worse for Muntz in Up.