What surprised Will was that he was prepared to answer no. The only death he actually feared was Elizabeth's, and he's happily diverted the blade to his throat without regrets. That doesn't mean he isn't delighted to hear the interruption, and to look up to see the beating heart in Jack's hand, a broken sword held above it.

(No sword turned by Will's hand would break so irregularly.)

His panting breath turns into sighs of relief, and he looks once again over at Elizabeth.

This is it. This is what he and Jack planned all along. Now all Jack has to do is stab the heart and it'll all be over. They've won.