Once the person is no longer a threat, the justification for the use of deadly force ceases. Any further act on your part can cause you a lot of trouble.

If a guy threatens you with a knife, demanding your wallet and you shoot him.......he goes down, say hit in the shoulder......and drops the knife, you are not justified in continuing to put rounds into him.

If he makes a move towards the knife......then he can still be considered to be a threat to you.