In my opinion, O-Ren Ishii is an iconic villain, and an absolute masterpiece of a character. In Kill Bill Vol. 1, she is one of the main enemies of the Bride. She's a highly skilled ninja samurai, a member of the Deadly Viper Assassination Squad (DeVAS), and the leader of the Yakuza. You really wouldn't want to mess with her.
Understandably, the Bride wants to kill her former colleague, O-Ren Ishii, as she was part of the group responsible for virtually leaving her dead on her wedding day.
Whilst the DeVAS massacre was truly awful and the Bride lost someone she loved because of Bill's vengefulness, O-Ren Ishii was purely following orders — something that the Bride would have done in her time, too.
O-Ren Ishii's character is also much more layered because she is who she is for a reason. After watching her parents die at the hands of the assassins who worked for Yakuza crime boss, Matsumoto, O-Ren made it her life's mission to avenge her parents' death and become the ultimate leader of the Tokyo underworld.
She may be a heartless and fierce character, but I think the complexity of her backstory demonstrates that she's not just an evil figure, but had good reason to end up where she did.