i would say 

stupefy

I think Sectumsempra, considering that:

-probably nobody except Snape and Draco( if he heard it while being healed) knows the counter-curse Vulnera Sanentur

-if you know how to use it you can kill if you want

-if you don’t know how to use it you will probably kill (*cough*Harry*cough*) if somebody doesn’t heal the victim

Besides the counter-curse to Stupefy, Rennervate, is well-known by most wizards.

Sectumsempra. 

Yeah, Sectumsempra.  A few others could be deadly as well (the Reductor Curse, etc).