Consider, If You Will

You are on your way home to your new spouse--who is waiting in your new home--after a long first day at your dream job. Life is, in short, perfect. Imagine that you are all alone, in a secluded, unpopulated area. It's late at night, and you stop and get out of your vehicle for a few minutes. While you're having your break, a figure lurches from the darkness and threatens you with a knife. In your extremity of panic, you retalliate with great force. In the exchange, the attacking figure falls. Hurriedly investigating, you find thatthe form on the ground is in fact a horribly dishevelled elderly man. From his dress, you assume he is homeless, most likely transient. To your horror, you realize that not only is he badly injured, he is now quite clearly dead. Assume further that you KNOW, with utmost certainty, that there are no witnesses, and no one will ever know what has happened.
Do you do the right thing?
What is the right thing?
Are you sure?