It highly varies. Just because a guy can pick up a rifle and follow some orders does not make them a warrior. I saw a high number of sheep wearing a uniform. Based on my experiences, I’d say 20% are the sheepdogs and the rest thought they could get some adventure and excitement and make some money along the way or get some schooling, but when the bullets start to fly all bets are off. After three tours I can say I had no negative effects, but I’m a Christian. Mileage may vart.
My opinions are not popular, but I’ve lost friends to suicide etc and I can only say knowing them, they weren’t mentally able to comprehend the job.
I have two friends up here I work on. Both of them claim PTSD, but I told them to turn their hearts and burdens to God.
The vast majority are what we used to call "noisemakers" and were never particularly proficient but they were steadfast and I never saw cowards.
There was a last group we would call "fodder" who had no skills or talent at all and never improved, They would stand on skylines, open gates, walk down the middle of trails, talk or smoke during night ambush/Listening post, etc. etc. Standing near them would get you killed.
But where I differ with you is that all three groups could suffer afterward: seeing friends blown up ( or in my case have a best friend captured, then murdered over a two week period) or having to kill somebody close up would affect anyone, even those with strong faith. Faith will help you get through but there is still a path that has to be followed.