Shen doesn't really count as a villain who became a villain because of some trauma. He allready saw dark things in fireworks before he got sent away by his parents. He wasn't traumatized at the time he destroyed the panda village. When he eventually got sent away, he was already evil as a *beep*
I remember reading somewhere that in the original script, Shen's parents did not like him because he's a white peacock, and because he was rejected by them he spent most of his time with the wolves where he was accepted, thus explaining why he and the wolves have such a close relationship. You can see when he returns from the panda village he is hoping his parents will praise him and recognize him but obviously they are horrified. The tragic backstory was removed and instead what we got was parents who allegedly loved him.
What past trauma is Shen scared by? SPOILERS: His parents used the power of Gunpowder to make fireworks that brought joy and prosperity their their city.
Shen saw them as a weapon and when the soothsayer saw his destiny he decided to commit genocide against the Pandas to prevent that.
He was pretty evil to begin with IMHO.
Yes being rejected by his parents for his crimes against Pandas may have hurt him,he was already a douche.
Making a movie villian bad just for the sake of being the bad guy is so bland. That's one of the reason why the villians in kung fu panda 1 & 2 work so well..... despite the fact that they're bad, they still have heart and it makes them believable and real.