That knife of Will's is going to come in to play during next movie..he's going to see it as his "way out" of the situation he finds himself in with Liz in the next movie...hence the fact he was angrily throwing it a the table whilst pondering over what he has just seen Liz do..it raises one thought in my mind:
"uh oh! someone get that knife away from that boy!!..If things get worse with Liz there's no knowing what he will do with it.."
well its obvious what he might try to do with it isnt it? "where is that heart of mine which is causeing me so much pain..? Oh there it is..now where's my knife?"