Trolling
Most of it is not malice. It is the format.
It is comforting to believe unpleasant behaviour online comes from unpleasant people. Mostly it does not. It comes from a setting where the person you are talking to is a name and a sentence, an audience is watching, and the reward for a cutting remark arrives faster than the reward for a careful one.
Anonymity gets blamed more than it deserves. Plenty of people are vile under their real names to an audience, and plenty are thoughtful under a pseudonym to a small room. What predicts behaviour better is whether there is an audience, how quickly the reward arrives, and whether the other person seems real.
Genuine bad actors exist and no design solves them — that is what moderation is for. But they are a minority of the problem, and treating everyone as one of them makes the majority behave worse.
What we try to do here. Sorting a response into disagree, depends or agree means naming your position before you start typing, which is a small pause in the right place. The composer asks how you would like to be responded to. Ordering responses least-read-first means being early or loud does not win you an audience. And when something does need removing, it is removed by a person, recorded permanently, with the reason attached.