Draco's failure at murder when he was a teenager might have been the beginning of a career where he wasn't trusted to get his hands dirty like the rest of them. Important missions couldn't be entrusted to someone that had a weak stomach and the inability to utter the killing curse. His skills as a tracker were very likely the only thing keeping him alive. She thought it unlikely that he hadn't been forced to kill for their master. But, perhaps unlike her, his thoughts didn't go straight to murder when it came to a pesky enemy.