Antisocial players and enabling DMs

PCs should at the very least be able to get along.

If a player wants to play an abrasive lone wolf who antagonizes anyone they don't like (and they don't like anyone) and responds to attempts to build bridges with them with insults then the DM should tell them to play something different.

I'm so tired of dealing with players who are clearly using their PCs to harass other players with the "It's what my character would do!" excuse (or don't see an issue with their fun coming at the expense of other players) and then the DM sides with them.

The DM refusing to do anything means antisocial players effectively get to force other players to do what they want since otherwise they can just claim the other player's at fault for 'causing the conflict.'

Rewarding bad behavior encourages more of it.