All of you are focusing on the moral aspect of this.
Full disclosure, my sons were not allowed to play these games, I found the act of shooting a realistic human to be something I didn't want my children to get used to. That being said that's my call, and I don't have impose my ideas on you.
The aspect of this you have all neglected, and does need to be addressed is this, this was simulator time for these mass murderers. Just like any Pilot sharpens his skills in a simulator to get better at what he does, these kids spent hours honing their skills on these games.
All of you who have spent hours on these games, can you say with straight face that you are not better prepared for a combat situation than if you had never played these games? No, they are not completely realistic, they might teach you some bad habits, etc. But, you are more capable in these situations than you would be without them.
What if you were a sick, obsessed, individual? Couldn't you use these games to try out different strategies, learn to be a more effective attacker? Keep in mind, if you are any better at these games than when you started you have already proved this point.
I've played various combat piloting games for years. I have a number of friends who are pilots that have played these games against me who assure me that I have a huge chunk of the skills needed for not just a pilots license but to be a pretty good pilot (they tell me there is a huge gulf between these two skill sets).
I think that there is no way you can argue that these kids weren't more capable because of their simulator time.
Do I think they need to be banned? No. I think something different needs to happen.
If you are a parent of a child who is struggling badly you have to make a decision about whether these games are a good idea for your kid. I think you should be held legally responsible for that choice. If your kid uses this knowledge to harm my kid, without any action to fight against the problem from your side, then you share responsibility for that action. I'd want strong enough penalties to induce fear in the parents.
I've had a child who was a serious problem as a teen. Until we learned that we were fighting a genetic mental illness we were stumped on how to fix it. We fought every day against the behavior tooth and nail. It was a losing battle most days. Sometimes the problems were stronger than the solutions. My child would have played one these games over my dead body. I'm not talking about a parent who is obviously trying to do their best but it isn't enough, I'm talking about the one who just doesn't care.
Gaily bedight, A gallant knight In sunshine and in shadow, Had journeyed long, Singing a song, In search of El Dorado