Honestly i think its pretty clear the office of the us president has become king, Rex non potest peccare. So i do agree with your broad point.
I just mostly think this is a particularly bad example of executive ignoring laws - using military force in armed conflict is not usually considered a crime and certainly not unprecedented. It is in fact very, very precedented throughout the history of the united states. There are circumstances where it can be illegal (war crimes, crimes against hummanity, etc) but generally the justice system around that is quite different than normal domestic laws around murder.
I would contrast that with some of the accusations against trump which are much less wrapped up in armed conflict and very unambigiously crimes (if you want an older example i would say the same thing about watergate)
We've probably reached the point where I bow out. But, on a related note, was the US even officially involved in the armed conflict in Yemen? I don't think there is such a thing as a declaration of armed conflict and my memory is the drone strikes were being kept relatively secret-squirrel. Obviously at the time everyone knew they were involved but I don't recall how official it got.
Declerations of war are not really a thing anymore in international law. Its not like usa ever declared war on Vietnam, but it clearly was one. Ditto for the iraq war. And its not just USA either. Declerations of war more or less stopped being a thing after the UN charter was signed.
You're right that that makes it messy. The us wasn't engaged in an armed conflict with the state of yemen, but with a non-state armed actor operating within yemen (and elsewhere). (And its not like that isn't true right now either - america bombed houthi positions in yemen just a few weeks ago. Different group but still a non-state armed group i would consider america to be in an armed conflict with). A lot of the ways we think about wars and what is just and unjust implicitly assume two states fighting each other. Its much more ambigious with non-state armed actors.
I'm guessing if Trump conducted a drone strike on the US Congress and claimed that the Democrats were associated with the Houthi you'd say that was illegal (correct me if I'm wrong). Would you say the major difference is the attack happening on US soil or would you draw a different distinction?