Their vessel did voilate the Neutral Zone (a possible act of war)
One wonders how this one works. If Genesis / Mutara was neutral space, and Klingons were spotted there, and it stood to reason that in order to get there they had to take their armed vessel through the Klingon Neutral Zone and thus violate its neutrality, but nobody saw
the Klingons at the KNZ... Would the Feds have a legal leg to stand on?
their vessel did fire upon a Federation vessel (another act of war)
Depending on what Maltz confessed to, the Klingons could claim that all they did was in self-defense. Kirk verifiably fired first, after all, and Saavik didn't see what happened to the Grissom
it's crew did murder Federation citizens etc...
Saavik did see that one stabbing. Then again, Kirk killed two Klingons on the planet, and blew up plenty more in apparent violation of rules of war (using surrender as a ploy is definitely something that gives the opponent moral and arguably legal justification for forgetting about the rules, too).
Not that Klingon law would have a priori
objections to killings of that sort. But we're not talking about law as a neutral entity, but specifically as a tool of politics.