When he was rejected being a sidekick by his idol, Mr. Incredible (actually being nice as possible about it by the way since Buddy was annoying), he was super petty and went full evil, killing all supers and then selling his technology to make tech supers and villains the new norm.

Better than the "ooh, I was wronged" or "ooh, I think I'm in the right and will make you think the same thing" that you see in villains like Zuko or Thanos.

He could have just been "Oh, if I can't be a sidekick to my idol, I'll just become my own hero, hoping to work with him or even being better than him one day!" But no. Buddy Pine/Syndrome is just a petty as hell rich guy who went full evil and that's why I think he's the second greatest villain in cinema history behind Thanos.