tl;dr - The most evil things you can do:
Take the Devil's Highway Perk (-2000 karma)
Blow up Megaton (-1000 karma)
Blow up the Citadel (-1000 karma)
Infect Project Purity (-1000 karma)
You'll notice none of those things are repeatable, however. If we use "donating purified water" as our baseline (+50 karma), repeatable actions that are at least as evil as giving away water is selfless:
Giving away FEV modified purified water (-50)
Giving Psycho to Paulie Cantelli in Rivet City (-100)
Enslaving any character with the Mesmetron: (-100)
Killing a non-evil character: (-100)
This wiki page lists all actions you can take to affect your karma. The full list of negative karma actions and their associated karmic impact:
Devour a corpse (Cannibal perk required): -1 Karma per corpse.
Stealing from a non-evil character or faction: -5 Karma per instance (note that when stealing from owned (red text) containers, such as cabinets or lockers, the Karma change applies once per activation of that container. Meaning, if you open a container once and steal two items, you lose 5 Karma, but if you open the same container twice, stealing one item each time, you will lose 5 Karma each time, which means you lose 10 Karma). Items can be replaced in owned containers after being stolen, and then stolen again and again with the same Karma loss each time, making for a useful method by which console gamers can decrease their Karma, although repeatedly hacking an owned terminal is a faster way of doing this.
Repeatedly hacking an owned (red text) terminal (Classified as stealing, but can be done over and over again) -5 Karma per hack attempt, NOTE, even accessing the terminal will result in negative Karma.
Selling ears taken from the corpses of good characters (you killed) to Daniel Littlehorn via the Contract Killer perk: -10 Karma per ear.
Killing a good creature: -25 Karma
Giving away Aqua Pura with the modified FEV with Broken Steel installed, to beggar characters like Carlos outside of Rivet City: -50 Karma per item.
Performing an evil action in a quest: At least -50 Karma
Killing a non-evil character: -100 Karma (although, this can be gotten around by shooting them once, making them hostile, then letting a follower kill them.)(note)
Giving Psycho to Paulie Cantelli in Rivet City: -100 Karma each time so long as he's alive.
Enslaving any character with the Mesmetron: -100 Karma
Causing Mister Lopez to commit suicide (either by pushing him or insulting him): -500 Karma
Letting the feral ghouls into Tenpenny Tower in the Tenpenny Tower quest: -600 Karma
Blowing up Megaton in The Power of the Atom quest: -1000 Karma
Infecting Project Purity with the modified FEV in the Quest Take it Back!: -1000 Karma
Blowing up the Citadel in Who Dares Wins in the Broken Steel add-on: -1000 Karma
Taking the Devil's Highway perk in the Broken Steel add-on: -2000 Karma (essentially, it resets your Karma to the minimum -1000).