I think it was a lot of killings, along with the fact that their coup failed, so he feels like it was all for nothing.
Plus he said it himself, he viewed all races with equal value. He lived, they didn't, and he has a hard time processing why they had to die, what more that he performed the deed on many of them himself.
It's a fair bit of character development. I suspected he was a former assassin on the original single-panel version, but author didn't elaborate on it very much. For a moment, I was afraid Giba-sensei was about to make him a former member of the gang that did all that to her. I'm glad that wasn't the case.