I was usually a GM, but when I was a PC, I was an absolute piece of shit. I would invariably create completely insufferable PCs who would deliberately annoy everyone. I would also act as a GM's foil, who would do things like betray the party to the enemy, or hoard information I knew that was advantageous to me, but detrimental to the party as a whole.
One of my favorite moments after betraying the party to the enemy and stealing from them was when I taunted the paladin of the group into just flat-out murdering me in a rage.
As a GM, I was fairly sadistic, too, although rolling behind the screen, I would quite often cheat in favor of characters who made the right decision but just got a shit roll. If it didn't advance the story, I'd just lie about it.