To change this, we have to assign a value to each single frag, rewarding those that target their peers. In multiplayer deathmatch, the quality of a kill does matter - the skill of your opponents matters. Jacob's Ladder Nobody really cares about numbers. The only thing the social animal does care about is rank. Your first attempt at a solution could be a ladder.
Players join by challenges, from the bottom, or by choosing an opponent.
Each time two players fight, the winner gets the rank of the loser, or keeps her own rank, whichever being the higher rank. Draws will be ignored. A ladder lacks all hints on how large the difference in abilities between two players really is, or any other linear scale of reference to compare players, thus making it hard to determine a worthwhile challenge. There is also no straightforward way two players could have equal rank. The ladder has a few things going for it, though.