One, make him a villain who happens to be a member of this group, and not that you suppose that all members of this group are villains. Describe his evil in ways that doesn't make it grow out of his group membership. His experiences as a member of the group may feed his villainy, but should not be the cause of it.
For example, I read a story once about a veteran who became a killer for hire. There's a point in the story where he talks about how the army "trained him to be a killer" so that when he got out he decided to "do it freelance". I found that highly offensive: the idea that learning to kill to defend your country from a hostile enemy would just naturally lead to killing innocent people at home, that there is any sort of moral equivalence between the two ideas. But I've read other stories with veterans who are murderers that did not bother me for a moment. If it says how he started out as a violent thug and joined the army because he wanted to be able to kill people legally, I'd buy that. Or if it said that he was so traumatized by some experience in the war that he went crazy, well now that's a cliche, but it at least sounds plausible.
As someone else said, you can avoid making it an indictment of the group by having a good guy whose a member of the same group. Most of the time, I'd say don't point it out. I mean, don't say, "Bob is also a veteran but he's a good guy". That's going to sound patronizing. Just put him there.
A couple of things I've seen writers do that I find patronizing. 1. Throw in a disclaimer. After you paint this group as soaked in evil and corruption, a character or the narrator says, "Of course not all members of this group are evil. Many are good decent people" etc. 2. Refer to the villains as "extremists" or "members of an extremist faction" within the group.
I've read many books and seen many movies that totally trash some group that I identify with, they portray every member of the group that you see in the story as evil and corrupt, they attack the group's goals and ideals, etc, and then when someone objects to the stereotype they blithely say, "Hey, we said it was an extremist faction, not the whole group". Yeah, you had one lame line about that, but the whole rest of the story made it sound like you were indicting the whole group.
Two, you get a lot less leeway if people closely identify themselves with the group.
For example, if the villain in your character is bald, I doubt that you are going to see a great uprising of bald people crying, "How dare you portray a bald person as the villain!" Because bald people don't really tend to think of themselves as a group.
This can split even on the same characteristic. Minorities tend to identify with the group more than people in the majority. So, for example, if you make the villain black, black people may protest. But if you make the villain white, likely no one particularly notices. Make a transgender person the villain and you'll be attacked as a bigot. Make a heterosexual person the villain and no one will notice.