I was in, and I did what I had to do to survive and fulfill my contract. It will be something you will have to live the rest of your life with. It feels pretty terrible knowing I sent someone to Hell instead of spreading the gospel to them.
This is nothing to romanticize about. Killing is ugly...
I shared a foxhole with a atheist. Believe it or not, but it is possible to get along without arguing. If an atheist isn't open to the Word of God, then the best thing to do is just live it as an example to them. This is where most people do things wrong.