I wouldn't kill him/her, just know that you have to add to its flock. Spend lots of time with him until his flock mates come along.
Although - it is super late in the season - it may be a long while before you get another fertile egg if that is your plan. You might be able to pick up a juvenile...
The above is unnecessary. Let it imprint on you and when it sees other ducks, it will go hang with them. Duck imprinting is not as strong as perhaps a goose. I have had human imprinted ducks take to their new buddies (other ducks) after hanging out together. They have so much in common - love...