Between Arenas, V'Ming spends his time as a lock laughing ominously in AV, tanking Olm with his own minions and pondering troll fashion from Zul'Aman. He's recently started to plumb the depths of SSC with his 0/21/40 build and bragging about 8k shadow bolts.
Sneaky, backstabbing rogues and warlocks seem to fit naturally into the evil mold - with the latter summoning demons from hell and all. Unlike rogues, who seem more like common thugs and hired hands, warlocks are more akin in flavor to evil masterminds, orchestrating doom behind the scenes and manipulating their minions to get the dirty job done. Incidentally, it was a warlock who got us into this Horde-Alliance mess.
Like Elizabeth, who wrote the rogues' version of "How to be evil", I'm not a mean, vicious or even evil person. In fact, if you met me in a dark alley, you'd probably invite me home to meet mom. So how do good people like you and me play this deliciously evil class to its full creative potential?
Any villain worth his or her salt will know the key to evilness is the Laugh. Not a giggle (too wimpy), not a guffaw (too oafish) but the full-bodied chortle of a megalomaniac. Let your victims know how much you love your work; let the Laugh come from within: use your diaphragm and tummy muscles. A throaty laugh just sounds too thin and insincere.
I understand that sometimes you may not have the benefit of expressing your mirth on voice, try these textual versions of the Laugh:
"MUAHAHAHAHA" (note the full caps and four reps of HA)
or even "LOLOL"
Never ever use ROFL, masterminds do not roll on the floor, laughing or not.
Very evil acts
Probably the oldest one in the book: summon an unsuspecting victim into the depths of the ocean. Convince your two helpers that they'll have front row seats to the game's funniest swimming animation, and slap Unending Breath on everyone (cast it on yourself first, naturally). A good drowning spot is the depths far west of Booty Bay.
Fear a questing lowbie into mobs. This prank works best in a crowded zone with mobs that are similar in level to your victim. Watch in glee as the mobs tear the victim up. You can seal his or her fate further with Curse of Exhaustion. To add injury to insult, pretend to help by waving, apologizing and putting level 1 DoTs on some mobs (preferably Siphon Life), then fear your victim again into more mobs.
Alternatively, fear the mob instead of your victim. This is more evil as it creates the initial impression that you are trying to help. If done right, you should train more mobs back to your victim.
Somewhat evil acts
Convince a mage to put up a portal to some far flung capital city. Ask your victim to click on the "summoning portal" to help summon a friend, he or she will be whisked away by the portal. This trick works best when the victim is drunk or otherwise intoxicated, or too newbie to tell the difference between a Mage's and a Warlock's portal.
Drain a lowbie caster's mana. Follow your victim around and keep his or her mana at zero. Other than warlocks, a caster without mana can't do anything. Very annoying.
Perma-banning druids in tree form (yes, you can banish them). Get a timer and start casting when the banish is about to expire, denying the tree-hugger any chance of switching forms.
Bring a few Warlock friends, head to a newbie town, summon some Infernals and let them go wild. Great way to spend a slow day in WoW.
Harmless evil acts (why bother?!)
Brag about how OP locks are and how big your shadow bolts are, constantly in guild or party chat.
Launch into an evil monologue for no rhyme or reason, telling everyone within earshot what your dark ambitions are, how much you despise gnome warlocks, and what you plan to do about them.
Deliberately pull aggro and death coil or fear a mob in an instance to send your tank running all over the place. Just a gentle reminder to your team on who's really in control.
In an instance, banish an elemental or demon target just as it is about to die. Apologize to your team, and do it again to the next mob.
Play the Warlock theme song over and over again on Vent.
If a newbie asks you for food and water, thinking that you're a mage, put 50g into trade and go AFK.
Important note for players seeking an evil Warlock career: you might find yourself alone in WoW, very alone, especially on PvE servers where everyone is supposed to play nice. Hey, nobody said that being evil was easy!
I'm sure you warlocks out there have more crazy-fun dastardly deeds to share; what is the most mastermind-ish thing you've done as a lock?