If you see your teammates fighting and they don't need help, LET THEM ****ING FINISH!!!!!!
Every time you either feel like you want to help or NEED that kill, you only make things worse by interrupting others attacks or hitting teammates themselves. STOP trying to kill all the enemies in the ****ing world.
Need to agree with Rooster here, it is madly annoying when you get interrupted by your own team mates and sometimes get killed because of it... But I usually just follow that idiot till he locks on to a foe, then stand right behind him and hack away at him to annoy the crap out of them until they get the message XD
People need to learn ganking techniques instead of just spamming side heavies, that is the real problem here