I would slay when there is like 1-2 seconds left or just slay him again next round. If he is on level 1 I would slay him when he gets off of it twice!
I think slaying can be avoided. Just freeze bomb them, that way the other person can still get the level.
If it gets to the final seconds and the guy keeps running, he is being a bitch and holding the rest of the server up. Slay the guy. If he wanted to be a bitch, he could have rejected the knife fight and fought with guns.