in the heat of the argument

All friendly debates start friendly, and most end friendly. But some reach an elusive boiling point. After this point, the debate is no longer about what you're arguing at all, but for some reason, about trying to hurt the other person as much as possible (under the mere disguise of intellectual masturbation). Every remark turns snide, designed to hit you personally where it hurts.

The worst thing about these debates is that you never thought your friend could be so mean and nasty to you (and probably vice versa). The best thing is that, once you move past the pain, you'll probably never argue again.