Often you can't even avoid teamkilling, someone runs right into your overhead, swing, thrust. Your are in ego perspective as an archer and someone runs through your haircross in the wrong moment and so on, and so on...
Surely you can watch several things, but sometimes it happens accidentally.
A proper solution would be: Teamdamage dealed*100 = Experience you lose. So if you teamkill someone intentionally you'll lose about 5000 experience. If you accidentally do the last hit its something like 1000