The problem with the concept of evil is that it is an explanation mechanism. Sure, innocent people getting killed is something really terrible, but it's not happening because terrorists are evil. It's happening because they think they're fighting for something good. For them probably we are the evil ones and need to be fought because we're not following god's laws. Even with your reasoning you could come to the conclusion that the west is evil, as lots of innocent people were killed in the Iraq war.