It's to fuck up the other guy. It's a valid strategy. And it's a good dickhead move. If you know you probably aren't going to get the next guy, getting rid of the enemy so he also has less of a chance to win is a good revenge.
The only time I don't kill downed people is if I know the other guy is right by me or is rushing downstairs from above. Otherwise, when I down them and I determine I have enough time to kill them, I take out my handgun or a shotty and waste them.
More points for me, less enemies, less chance of others winning. It's win/win for me no matter what.