I think I'm going to go with: People who deliberately try to ruin other people's gaming experience.
Quite a broad answer, I know, but it was the only way I could cover griefers, mini-game ruiners, crashers, scammers and so on.
If you only do what you can do, you will never be more than you are now.
When people leave their grocery carts right in the middle of the parking lot of a busy shopping area.
"Revenge...is like a rolling stone, which, when a man hath forced up a hill, will return upon him with a greater violence, and break those bones whose sinews gave it motion."- Jeremy Taylor
People who like to skill right next to you when there is no need to. For example a few years ago I was mining in the mining guild and it was just me and 1 other player. They felt the need to come and mine the same coal rocks that I was when they had the whole guild to mine in. When I moved areas they followed.
Also people who call me a noob because I "only" have a max cape
Smoke me a kipper. I'll be back for breakfast.
The PvP community is a pretty toxic place as well, but my most memorable experience was an encounter with a really braindead Abby crasher. Who wasn't apologetic in the least. My clan came and actually anti-crashed, but he still remained there for an extra 30 minutes flaming us.
He was so annoying I actually added him to my ignore.