This is actually a very difficult argument to refute in some ways.
The most likely way to refute it is to say that there are natural laws that are so basic that they apply to all human beings at all times and places. Therefore, it does not matter if there are no specific international laws. There are some things that are so clearly wrong that you simply cannot possibly think that they are right. Therefore, it would have been ridiculous to let the Nazi leaders get away with genocide just because there was no law against it.
However, this is a hard argument to make. Just because “everyone” believes something is right or wrong does not make it so. “Everyone” felt that slavery was justifiable and yet it was wrong. “Everyone” felt that it was wrong to be gay and yet many people would now argue that it is not. It is hard to know what so-called universal laws really are universal.
My reaction would be to say that this was the best that we could do. The leaders had to be tried, not just summarily executed. Just killing them would have set a bad precedent and would not have seemed just. Therefore, a justification for putting on trials had to be found and this was the best one.