I LOVE Scrappy so much that when they find out that he's evil in the live action movie I don't think of him as Scrappy, I think of him as some OTHER evil dog I made up called Crappy.
he was only made evil in the live action movie as an attack on the character. it was totally out of character for him and in other versions of the show, he was actually useful.
Him turning evil was a more drawn-out process that a lot of people seem to have forgotten about... like an old network bumper for Cartoon Network in the late 90s/early 2000s showed he was becoming more and more disgruntled having to take a backseat to more modern characters like Johnny Bravo and Dexter, among others. That might as well have been his breaking point.