Here's a totally crazy idea...

When you see that a teammate is about to kill an enemy but running low on health, instead of rushing in to secure the kill yourself, engage other incoming enemies and stop them from interfering and killing your ally. 90+% of players will run in for the kill, feel proud of themselves for a brief moment before their ally gets mowed down, then chase after the culprits prior to getting quickly mowed down themselves. Be better than 90+% of players!

Bonus points if instead of using your CC on an enemy that's obviously screwed anyway you use it to stop or slow down would-be interlopers.