That we should strike before North Korea does

I would basically have argued that since the probability of North Korea striking at all is quite low, given its very limited resources and few allies, a pre-emptive strike on them would not only be an enormous waste of resources on the part of the US (which is who I'm assuming "we" is referring to), but it would also be highly unethical-- it would start a war that would otherwise never have happened, meaning that all the resulting casualties would essentially be unjustified murders inflicted directly by the US.  Pre-emptive strikes in general need a very high standard of evidence to be justified, and so far North Korea has done very little to imply a significant threat of attack. 

Analogy: 

A scrawny teenage boy is taunting a 300 pound man at a bar. In response, the man pulls out a pistol and shoots the boy in the head, claiming that he had to do it stop the boy before he started assaulting him.

Res is negated.