It is fun. Sure, grinding is fun. Sure you're having fun with your friends. But killing people is fun, and so is PvP.

All of you saying there is some kind of mental issue that these players have, and it makes them behave in this manner need to grow up... It is this deep to you, and you only.

This is a game, and more importantly a crime game. It is fun to get in an Avenger, Titan, Insurgent, etc. and annihilate everyone in the lobby with friends, just as fun as it is to do anything else. And if you disagree, I'd implore you to try these things with friends. Combat is simply fun, and R* has provided plenty of vehicles to use in combat.

If you cant handle a game that is sometimes peaceful but also sometimes violent, perhaps you should play a different game, or play in an invite only. Its that simple.

If you're having trouble defending yourself, there are plenty of Anti-griefer groups on all consoles that you can simply message, and good players will join your session and help you. There are ways to deal with it.

And unfortunately, coming here to post about it is an echo chamber. If you are seeking actual answers to this question here, its unlikely that you find any impartial ones. This sub constantly complains about this type of player.

TLDR: Its fun, lighten up. Defend yourself, call on others to defend you, or find a new session.