Replacing my brother's San Andreas custom music files with Justin Bieber.
That and pretty much most of the nastier options in the like of Kotor II, it puts even killing Mission to shame in the first game when you can psychologically break innocent people, crush lives like dry petals, sell people into slavery, torture, lie, corrupt and manipulate most people you come across during your journey. The dialogue and development of relationships throughout the game really do separate you from the 'puppy stomping psychopath' kind of evil staple, as I progressed through my dark playthrough, it really hit me at certain points that even I thought my character was a monster, that and being a walking harbinger of oncoming desolation is more than a bit unpleasant.
I would concur that Chris Avellone's work on games such as Kotor II and Fallout New Vegas are a fascinating web of complex moral dilemmas and consequences of actions, the latter game is indeed full of potential for your character to become an atrocity of human nature, though I find the Legion isn't necessarily the sole representation of said evil actions; and as many have pointed out in this thread, nothing quite like munching on a few humans.
Though it is at times fun to act up the psychopath for mindless evil, one of my fondest memories being when my character, having felt inspired by Vulpes' work at Nipton, decided to emulate his figure of inspiration by walking into Camp Mccarran, thoroughly tooled up of course. Whereupon he produced a light machine gun and began spitting hails of bullets at the surprised NCR troopers that were on duty, it being just before dawn meant the majority of the camp received a rude awakening, so with the bullets thundering out, mowing down a number of guards, the few that survived the attack attempted to retreat, hobbling on broken and crippled limbs. This being for my character the equivalent of spilling a barrel of blood into Shark-infested waters, prompting him to stow his ranged weaponry and pull out his trusty Super Sledge, and with it he gave chase, cornering his wounded targets one by one and bringing down the Super Sledge upon their skulls, beating and bashing until the corpses were left as bloodied chunks and pieces, whilst relishing the screams that came with the bludgeoning.
Once the task was done, he decided to leave a mark by gleefully dragging as many of the bodies and messy pieces of human flesh as he could back to the front gate, and arranging them into a pretty little display that resembled a simplistic smiley face, one that left a disturbingly childish effort at art, albeit marred and perhaps changed by the use of mutilated corpses. However, he was not completely heartless or conservative about his work, for he graciously left a single 9mm pistol with a box containing just one bullet, neatly placed in front of the organic piece, so that any remaining NCR comrade to return to camp and discover the work would be able to end their suffering, and contribute to the work.
Said character would later become the darling of the Legion for his artistic efforts, even if he was later suspected of having sliced up his own inspirer with a ripper and hiding the body parts in the Ultra-Luxe front fountain, presumably out of some misguided sense of jealousy.
I fear I may no longer be in control of him...