Many of our high school campaigns ended in interparty destruction. Or at least betrayal that resulted in destruction.
Two of the most memorable moments were in 1e/2e when the party was exploring a lich's tower that was supposed to be vacant only to encounter the lich returning from a planar journey of some kind. We tried to fight for a couple of rounds but he dropped one of the PCs quickly with his paralyzing touch. The rest of the party fled down a corridor that ended at a locked door. The rogue starts picking the lock and then the rest of the party stands and fights to give him time. While we are fighting the lich, the rogue opens the door, runs through with all the party treasure sacks, and then locks it again from the other side, leaving everyone else to die.
We are screaming, "Open the door!!" while the rogue's player tells the DM he is putting contact poison on the lock in case anyone tries to open it, then he flees out of the tower leaving us to die. We do all die at the ends of the lich. Even the DM was kind of mad because he had intended for the party to escape.
But in a brilliant bit of counter betrayal, my wizard who secretly had an invisible imp familiar, regenerated back to life and escaped after the lich had left (the DM ruled that the regeneration granted by an imp to its master would function like a ring of regeneration and even bring me back from death). This was all done through secret note passing so no one but me and DM knew that my wizard had lived and escaped.
Next session, everyone had made new characters and were deciding whether to add the rogue who escaped from the prior party into their new group (players did a good job of role-playing their characters without previous knowledge of the betrayal).
Then suddenly my wizard shows up and disintegrates the rogue for his betrayal. It was awesome! Even the rogue was like, "But, you should be dead!" It was a classic revenge scene, like out of a movie. I happily retired that character after that.
The next classic bit of interparty fighting happened in a Dark Sun game that I was DMing. Two of the players hated each other. One was playing a treacherous rogue (the same player from the prior story, big surprise there) and one was playing a half-giant gladiator (the DM from my prior story, who still bore a grudge at the unstable rogue player for messing up his prior campaign). The rogue was his typical treacherous self, constantly stealing from the party, pocketing treasure while the rest of the group was fighting and so on. There was already some thought of him getting booted out of the group.
So in one portion of the published adventure, I think it was Black Spine, the party frees a mind flayer from magical stasis, but they don't know what is because they aren't normally found in Dark Sun. The flayer quickly dominates the half-giant (2e psionic rules) and commands him to destroy the party.
"With pleasure, master," the half-giant's player said with an evil grin and immediately attacked the rogue and pretty much wiped the floor with him. The rogue's player was pissed but the half-giant player thought it was great.
Suffice it to say, the rogue player hasn't gamed with us in over 10 years.