Modern Warfare uses a system based on performance.
In team deathmatch you get awarded bonus points for things like "payback" where you killed the enemy that just killed you, or a "buzz kill" which is you killing someone one kill short of earning a "kill streak".
In Team Deathmatch if you perform a regular kill you get a base score of +50, depending on the settings... and if you save a team mate in the process an additional +50 is rewarded for being the rescuer, if the bullet that kills the oposing player is a head shot, throw on another +50.
That is a +150
So depending on the circumstance of the kill, you can get a variety of tallied points for a single kill.
You are rewarded more points for accuracy, revenge, mission focus and so on.
Some of these factors are to motivate you to be positive despite possibly being repeatedly killed while learning.
Others are obvious rewards for just being great at the game.