Actually, in a related idea, I thought "how about rewarding players for not killing anyone?"
If you haven't killed anyone in RL day, you get 10 experience points. Then, another day passes and you get 20 experience points with a text "You discover that not shooting people on sight is also possible."
After three days you get 40, then 80, 160 after five, 360 after six, and on the seventh day you get 500 experience points.
"Your peaceful ways allowed you to gain a lot of experience by simply talking and interacting with people instead of mindlessly slaughtering them."
And each day after those 7 days you get 500 experience points until you kill a player. Then it is reset and you start from 10 again.
This is a fairly small amount of experience points. You'd actually need over 40 days to reach level 21 this way, and if that is too fast, then maybe you are awarded experience every 24 hours of play time.
The bonus is small, but psychologically I believe it could be enough to prevent some people from killing just anyone. A slight "if I kill that guy for no reason, I'll lose my 500 experience points..."