War is an affair of either kill or be killed. What veterans most commonly say about it afterwards is that they did their duty and what they had to do. In this case, it should be "easy" in the sense that they will indeed be fighting against capable opponents very willing to kill. The last time the "heroes" (all of them still on the human empire's side, save Nakatani) were attacking women and children. Some of them seemed to be glad to do it. Even if such "heroes" were now opposing the ones who switched over to the demon side, it shouldn't make anyone to lose their sleep to get rid of them permanently.
All in all, I don't think it's of any use to keep talking about it before their very first real battle. The battle itself should teach them. When someone is trying to kill you, it should be easier to accept killing that person first, compared to just theoretically talking about it before anything has happened. Like Rambo said: When you are pushed, killing is as easy as breathing.