The Goomba, Ghoul, Zombie, Locust, or whatever else the last enemy you killed was, had a family. A wife, three kids, a white picket fence, and college tuition to pay for his oldest. You now have to deliver the news of his/her demise and tell the family the reason he/she was killed.
Can you do it? Look them in the eyes and tell them you killed their loved one for a game?
Note: No you cannot just kill the family and continue on your way, nor can you burn the house down.
Edit: Even if you think you killed every last bit of his family there is still at least one person left. Could be a old lady could be a new born point is there is always at least one left you have to tell.