Note that I believe there are legitimate cases in which a character has every reason to do something detrimental to the party, as their most reasonable option. These cases might not be apparent during character creation, so they might arise without any malice on anyone's part.

That said, when a party member does something to hurt the party, they should remember that it's totally reasonable, in-character, for the rest of the three to five other skilled, heavily armed mercenaries to exact retribution. Would your thief really think it worth stealing a few minor magic items from the fighter, knowing full well that the fighter would probably end up finding out one way or another and could easily break his teeth in (not to mention the fact that the thief probably relies on the fighter to save his skin at least once a day)? Is that something most people would do?