In some contexts competitive back-and-forth insulting is a baked in part of the culture, and is mutually enjoyable. You're only a jerk if you can see that the other person is no longer enjoying it and you continue anyway.
Trash talk is also not the same as engaging in trolling. Telling someone you’re going to kick their ass is perhaps reasonable trash talk in some cases. Calling someone racist or sexist epithets is not.
Please don't take my comments to mean that insulting/mean behavior is OK, though. IME many commenters across the web are far to quick to hurl insults in situations where they have no idea how they'll be received.