First you have to figure out why she can’t kill him. Is it because you have grown too attached? Has she fallen for him? Is it not in her to kill someone? Is one not yet angry enough to force the other to make a move, or make one on their own? Or is the story just not there yet?
Also, think of some other possible endings. To ensure he’s not killed, he kills her. Or, she falls for him, finds out something not so nice, then kills him. Or, she falls for him and he accidentally dies by some other means. Or, she gets killed or dies accidentally.
Isn’t it fun?