In general international law is much more lenient than people are willing to believe. e.g. it's legal to kill civilians if you are attacking a military target which is important enough
Once they declared it a terrorist organization (which is the problematic side of everything), they can claim these are unlawful combatants and do not have any of the protections of the Geneva convention, like any other war on terror assassination.
So I don't think double tapping is a war crime, any more than bombing a car with terrorists in the first place and that doesn't seem to be regarded internationally as a war crime. However, they could have done better to highlight Venezuela actual involvement with terrorism (which is real but not enough for this) rather than magically declare them terrorists just to not go through Congress
That "unlawful combatant" designation was invented by the US as an excuse and has always stood on shaky legal grounds even in the US. Other Western countries don't support this legal construction. That being said, the double-tapping was ordinary murder, not a war crime. Every bombing of those ships could have been avoided by boarding them and presenting those drugs as evidence, as the Coast Guard normally does. But that would only have worked if there had been any evidence to start with...
There are some credible war crimes accusations (in fact, some pretty flagrant war crimes), but the most critical crime is actually not a war crime, but one precedent to their being a war at all, the crime of aggression.
Starting a war is generally what is known in modern international law as the crime of aggression (in the language of the Nuremberg Charter, this was, “crimes against peace”, the first listed category of crimes subject to the tribunal, above war crimes and crimes against humanity.)
Rudolph Hess, notably, was convicted and imprisoned for life solely for this crime.
