I wrote a simple mod which makes players want to kill each other using a points system.
There are one or more flags, whoever who has one of them in his inventory earns points every now and again, players who don't loose them - but that happens only to connected players when 2 or more players are connected, so you'll never be earning points without someone trying to kill you, or loosing points without someone to kill.
When you die, if you had a flag, it appears where you died, and you loose it.
I could put a screenshot of the flag, but it's just a simple blue nodebox, so I think you can imagine it :)
Code is under the AGPL v3, textures (a 16x16 blue png) are WTFPL. Oh, and it uses this, which is not mine.
Downloads:
v0.1: http://ompldr.org/vZmloeQ
v0.2 http://ompldr.org/vZmlqMQ