Even with those that do see combat taking a life is not something that most (in my experience) take lightly. Often is kill or be killed, and in the case of snipers they often have to take one life to save other lives. Again, it's very easy to judge if you have not been in that situation.
It's a complex thing. No, people are not automatically good or beyond reproach if they are soldiers, but I personally give that occupation more respect than say, a hedge fund manager. That doesn't mean that I give soldiers a pass on bad behavior, and I honestly don't think most people do. I also feel that the mental toll that combat takes on people must be incredible--particularly in a conflict where there is no clear designation of who the enemy is.
I can't even imagine what it was like for those Vietnam vets to be sent, often without a choice, into a hell halfway around the world, see unimaginable horrors, then come home to an ungrateful and downright nasty reception.
Honestly, if some random person walked up to my step brother or cousin criticizing their service I wouldn't punch them (not really my style), but I would have a very strong suggestion for where precisely they could shove their opinion.