This is honestly a fair question, as this Grail War wasn't anticipated to even occur, we have no idea who is supervising it, and we're not even in the city where the grail seems to be. I'm not sure if all of the rules of past wars even directly apply to this one - and Lily wouldn't know the details of any of them anyway.
>What do we know about the rules regarding elimination of other masters, for it to actually 'count'?
>For that matter, do we have any idea how we might find out who is supervising this war?
But there's the additional issue that even if Koch isn't a candidate to win anymore, it might nonetheless be in our best interest to kill him. He has more than enough money at his disposal to cause further problems for us (and is possibly motivated to do so) and it would probably be good if he couldn't provide testimony to the police that would implicate us in the death of that police officer or any other part of this mess. Though I also find myself wondering if there's anything useful we wrestle out of him at this point. I don't think we have any means to take his command seals, handy as those might be, but he still is definitely more knowledgeable about the players in this war than we are, if he knew where to find us.