If you see a friendly on the 6 of an enemy with an enemy also on the friendlies 6 - ALWAYS opt to kill the the enemy which your friendly is chasing and ignore the the enemy on the friendlies 6 - that way you can maybe get a kill then both get killed by the guy you chose to ignore.
If you enjoy giving the enemy 2 kills for the same effort to kills 1 guy - i highly recommend you do this.
OR - to put into a perspective that maybe majority are only able to comprehend, let me change the perspective:
If i see YOU chasing and shooting an enemy and YOU have an enemy on your 6 - when i get close to help - i will choose to ignore the guy on your 6 and instead kill the guy you are shooting, after which - we can both get killed by the guy on your 6 i chose to ignore. :)
I know majority do this anyway but i just wanted to reinforce this highly effective and logical method of being an utter detriment to any team you ever get placed on.