It's certainly reasonable to expect that there are at least some mentally ill people in any decent-sized community, but the Greater Internet Fuckwad Theory[0] suggests -- and I agree -- that there are many people who just turn into complete assholes when they are anonymous or semi-anonymous, and can hide behind their computer, tens or hundreds or thousands of miles away from the people they interact with.
I don't know the demographics of your game, but this is especially true of teenagers. (But not exclusively true.)
[0] https://www.penny-arcade.com/comic/2004/03/19/green-blackboa...