When horrible things happen and we feel powerless to do anything about it, that stirs us in many ways. Anger. Frustration. Hate. Powerlessness. We don't know what we should feel, although we suspect that dropping a nuclear device on the people who beheaded an American seems like a good idea. We know that isn't the answer.
What is the answer. First, don't lose your ethical balance. Continue to evaluate everything with the same ethical yardstick you used before this happened. Don't kill all, even figuratively, to be rid of the "confusion" about what civilized people will or won't do. Don't assume that only some civilizations do not have deviants in their midst. We all have our share of humans that seem to us to be uncivilized.
Talk to somebody. Not about what you've seen, heard or read, but about how you feel about what you've seen, heard or read. Friends are great for that. If you can't find a friend, find an advisor, councilor or anyone who will help you talk about your feelings. Don't keep them inside. You really can't. They will come out and you may not like what you see when you explode.
Give yourself a chance to absorb the event and keep expressing yourself as you do. You will be able to cope.
That is no way changes the horror of what happened. You're the one that adjusts. Now you're free to move ahead helping others cope with what life dishes out.