Personally, I don't see a hint of evil about quickly killing someobody who plans to murder several thousand people, just in order to prove some twisted point. I think the line was intended to show a character changing moment for Alex, that he is becoming "cold" about his job, but really... what would be so bad about killing Yu?
Does that make me sick?

But he killed in anger, not in coldness. He probably wasn't thinking "Well, if I kill Yu than the least amount of people would die." It was probably more like "That sadistic jerkwad deserves to die! And I hate him!

"