So picture this. You're the leader of a small group of 6 rebels against a fascist government, you have you have 4 people that fill generic roles of somehow trying to...well...resist. You also have an immensely talented doctor and field medic who's really good at all kinds of healing and medicine (that and futuristic tech). And then there's a 6th member. This guy is like everyone else. But he's also a little bit more mentally unstable, and he also took part in a school shooting, killing many many many people. Though they were all pro-government irredeemable assholes, and his partner was kind of the leader and he was the follower who may have used him, he still killed people. What would be a realistic reaction to him meeting the team? He has the same beliefs, just he...did that. Would they work to get the doctor to treat him first for his obvious mental issues? I'm also thinking for an alternate scenario, a villain maybe stops the shooting by killing the kid's partner and kidnapping the kid to be her little bitch.
You know, not everyone who picks up noble causes have noble intents. I think a lot of people who pick up the cause for Social Justice (a respectable cause) for example, don't care two wits about impoverished minorities. What they like is the feeling that they get from intimidating others and calling them Nazis and all the thumbs up they get on Twitter from saying how they will kill any racist they see. Social Justice just seems to give them permission to bully. And so naturally a lot of bullies love it, which is sad because it gives real Social Justice Advocates who do work hard improving the lives of others and do respect other people's opinions and don't want to waste time on witch hunts such a bad name. So that's one way to look at your character. Maybe he doesn't actually care as much about making a better world as he does unleashing his anger. And it just so happens that this rebellion gives him permission to do so. With that said, I would probably only tolerate him. I wouldn't like him. I wouldn't even make that dislike a secret. He would know I don't like him and trust him. I could imagine people in that group having their fingers on the trigger at all times when he's around and will take him out themselves if they have to. As for the doctor treating him? No. Mental Problems that lead to violence are very severe. They would spend more time treating him then the use they would get out of him. So, that makes no sense. I can see him going to the doctor privately and asking the doctor privately to help him. And that could create a sense of empathy from the doctor. Unfortunately, the doctor won't have the necessary time to spend to properly treat him, so his condition wouldn't improve.
Different people would react in a different way. You know your character best. You know how you want him to appear to the reader. So your next job is to convince the reader that this is how such a person would react. The exact reaction doesn't really matter. The execution is highly important, it's about convincing the reader it actually can happen like that. His reaction is only a tool for your narrative. If you want him to appear cruel, he could burst out laughing, if you want him to appear wise, he could nod head and stand there deep in thought, if you want him to appear brutal, he could start shouting. He could do anything you need him to do.
If it's the "small band of plucky rebels against the evil government," they'd most likely tolerate/use him as @Kallisto said above. Think of other fictional examples, like Con Air, where there's a mix of "honest" convicts just trying to do their time and get out along with "evil" cons like the rapist Johnny 23 and whatever child molester Steve Buscemi played. All of them have to work together in the common goal of escaping. For a (more) real-life example, look at the cable miniseries The Pacific. That's based on a couple real memoirs, but I don't know how closely. Still, some of the Marines are taking the gold teeth right out of the heads of Japanese corpses, and in one case, out of the mouth of a man who isn't yet dead. The Marine doing that is interrupted by a buddy who stops him on moral grounds kills the wounded Japanese soldier first since he apparently sees the live extraction of a tooth as immoral, but has no problem violating other portions of the Geneva Convention. Point is, there's always going to be a sliding moral spectrum in a fighting group, and it could even serve as a source of additional dramatic tension for your story.
Well, I don't think I would trust him completely. Having said that I'll be polite to any one and although I'm not a genius I don't think I'd join only 5 rebels against the Government without a damn good reason. I'd be looking for more people to stand a fighting chance. But then knowing me I'm such a procrastinator I'd probably never get round to joining them in the first place haha.
