Did all players agree beforehand that PvP was OK and that acting against other players fine - without being pressured to do SO? If yes, well OK, though honestly not my preference.
If not? Then this is horribly jerkish behavior. And, since the other players were surprised, it seems no consent to PvP was discussed or given.
The guy essentially ruined a successful campaign run by deciding to massively hog the spot light to be the last person standing. The fact that he planned it the entire time and threw in "little clues " along the way to essentially troll the party? Just wow, sorry this guy is NEVER getting invited back to a session if I'm the DM.
The "it's what my character would do..." end after pointing out these clues is the cherry on top the jerk top! "ha, ha you should have seen it coming..." is exactly what a jerk says to justify this kind of terrible behavior. Again, if the group had agreed the characters could /would possibly murder each other, will OK. But even from your short description, it sounds more like the guy was just unilaterally imposing his fun on the table - which is not great.
It was never discussed. Those D&D campaigns lasted 3-4 months. And, here is the key: We are all playing drow - in their traditional sense. The setting was Underdark. If you do not have one that is going to backstab the others, then you are pushing aside thousands of pages of lore. I would ask in any game: should you toss aside culture, traditions, and backstory - just to make sure there is no inner party conflict?
That is the new thinking though; my githyanki plunderer can be in a party with a githzerai Buddhist style monk who is in the party with the lawful good dragonborn paladin who is in the party with chaotic orc from the Tribe of Many Arrows who is in the party with the halfling clepto thief and everyone should still get along - for the sake of the table.
Let me give you a hypothetical: It's a traditional setting. Forgotten Realms. A rogue steals from a dwarven clan. Nothing big, but some gems and a nicely forged thing-a-ma-jig. The rogue does this because he is following his flaw straight from the PHB: "It's not stealing if I need it more than someone else." The dwarf in the group finds out about this. It was stolen from someone in his clan that he had in his backstory. Someone, because he is playing the dwarf as described in the PHB, will defend his clan at all cost. What should happen? Should they ditch the lore, the backstory, the ideals and flaws they wrote for themselves? Should the DM have to check in session zero everyone's bonds, ideals, flaws, personalities, alignment, and backstories to make sure everything will remain nice? Should the dwarf be allowed to try and beat the tar out of the rogue? What if the dwarf gets his clan to have the rogue arrested, and now the PC has to sit in a prison smacking rocks for the next month? (Could be handled with a cutscene, but sometimes not.)
For the record: the players at that drow table consider that to be one of the greatest RP moments they have ever seen. They still talk about it with fondness. No one was upset or hurt in real life. Maybe because we are all over the age of 20 or maybe because we all have a lot of experience. I don't know. But I do know, no one was upset.
This seems to be joking around, and while not everyone's cup of tea - it seems the table didn't mind it /had fun with it. Also, this didn't generate conflict, so much as eye rolling. And the player wasn't being a jerk because he didn't continue after the joke had run its course or make anyone uncomfortable.
Yes, everyone, including the DM, was fine with it. Jokes get old. There doesn't need to be some social contract to explain this. It also needs to be recognized that jokes can come back and be funny, even after they get old. It's all in the timing.
But the most important fact should be that it was okay for this table. For another table, it might not have been. And that is okay.