You are trying to use the social construct of "international law" to justify your position, your position being "the U.S. shouldn't have curb-stomped a terrorist on somebody else's lawn", which in my eyes is a minor complaint. I don't see the difference between the two, an appeal is an appeal. Soleimani was a key instrument in the Iranian regime, he orchestrated a number of attacks on American and international troops over the years, and he didn't seem to be bothered by international law or severeignity either. Sure, it would be nice if the American intelligence informed Iraqi intelligence about the operation, but there are several problems here - one, they would risk blowing the whole thing if anyone spilled the beans, two, it would directly implicate Iraq directly which could possibly lead to another war whereas now they can just wash their hands off the whole affair and three, they had to act fast. Once again, as far as I am concerned, justice was done and the Iranian regime got just a little bit weaker. Quite honestly, I much prefer those smaller, covert strikes over full-blown land, sea and air warfare that costs trillions of dollars, causes thousands of deaths and leads to untold collateral damage. Those big, pompous displays serve nobody - not the indigenous population nor the international community. I don't see anyone crying over the general, nor do I see a war going on over it, so there you go. For once the U.S. avoided using drone strikes accidentally aimed at weddings, sounds like a win to me.
