I'm very respectful to people online... usually. I even have trouble talking online, just like real-life. If somebody sends me a friend request, I'll accept it out of guilt or not to dissapoint them, ect.
But really, the worst thing I've ever done is in Minecraft, after he robbed my house, I filled some guy's house with obsidian, stole all of his pickaxes, rounded up all of his animals and books, and burned them all. I then just kept throwing splash potions of slowness at him as he was trying to come at me with a sword. It was amazing. Stupid griefer.
It was hilarious.
For an MMORPG, Runescape. I'd generally either annoy or confuse people why my sheer strangeness. This actually stems from my real personality, which is very different from the personality I try to show whenever I'm online. In reality, I'm a very naive and sheltered person, and I try to keep the two personalities apart.
The fact that I got an entire guild of about ~90 people pissed probably tells you a lot about me, simply shows you a lot about me. I didn't even do it on purpose, it just happened. Somehow.
The only time I did it on purpose was following people; y'see, Runescape has a "follow" button, it means that you can basically join the loosely termed "party" of that player. In other words, conga line. I'd follow new players, and when they were in fights and dying, I'd just sit back and watch. The things that I were called were hilarious.