Fun but evil things to do:
Get a high lvl Hammeridin, and enter a Baal run game. wait till everyone is baaling, hostile and tele to them. They will all die, or run out of the game.
Do cows with a high lvl Windy Druid for all those trying to lvl up to 24. Run around them atracting all the cows and then, tp out to town, watch everone flame you.
Q&A for items. At the end of every ladder season, I have a Q&A room where I ask a question, and the first person who gets it right, gets a free item. I only keep 1 or 2 characters with their end gear, so everything else goes.
Go to a trading room, and tell someone to create a privite room for trade since all your gear is on a mule. Show up, and tell em you'll be right back, but get his character name first. Then go to another trade room and get someone else to go to the first privite room, where you will be waiting for the item you were suppose to trade for. This creates a bit of confusion between the two players. I did this as an april fools to some players.
create a hardcore character and lvl it with a priend in a privite game. Then go and PK some noobs in a open HC lvling game.
Just out of bordom, do a baal run game, but act as a bot. Then after so many runs, confuse the hell out of them by asking who the bot is.
Make a clan called KKK, and use nothing but pallies. (Uhhh they are black guys)
Join a low lvling game with a high lvl character and kill all the act bosses then leave.
This takes some work, but try chatting backwards in the chat rooms, and games. Or simply join a game, and say "long time no see (make name up)" They will get so annoyed at you for calling them by the wrong name that they will ignore you, then talking in the bubbles, tell him that "I guess you don't want that Nigma I owed you then" and leave. This will piss em off so bad he will msg you the rest of the night while kicking himself in the ass for ignoring you.
enter a game for new character lvling, and tell them all you hid something in the room, watch em all scramble to find it.
Try to pretend to cyber someone playing, they get annoyed fast.
start a rumor see how far it goes
When someones character dies, drop a worthless item on their corpse, if done right when they return to get their body they will leave behind an item they were using instead.
In a HC game create a suicidal character, join a game with a friend, but give the frined the opten to grab your gear from your corpse. Then wait till everyone is battling by a waypoint then go into town and hostile them, ifthey are high enough in character lvl they wont even bother to see if you have facets on because of their high lvl. When they attack you and them die. Arrogence at its finest.
give some noob maphack, then tell them to activate it you need to press 5 as they enter a new game, this will cause them to kick off the room, when they come back, say " what happened you did something wrong, try again. " After 4 or 5 time BNet will thing they are a bot and lock them from joining a new game for a few mins
befriend a PKer, and after they have trusted you, they might ask you to help xfer, or hold something. Take it and run lol.
be as annoying as possible when lvling in a full game.
help someone do the Mephisto quest, but after they kill him, hostel them till they leave. If they have not gone into the portal to act 4 they must kill Meph all over again for the quest.
Uhhh thats all I can think of for now be back later with more.