A green card is a residency card; it does not confer citizenship. Lots of Journalists with American citizenship have residency cards of other countries in order to be able to work from those places. It does not make them Japanese, or Korean or Chinese, etc.
> They did murder an American journalist for criticising their rules in his writing
In your quest to bust the myth that they killed an American journalist you are missing the point. The point is that they did murder a journalist. Here I am going to extend this even further: they murdered another human being.
This reminds me of a great exchange in a classic movie: https://www.quotes.net/mquote/9306 The exercise to map this to our discussion is left to the reader.
this is what it means to be a sovereign state - they _can_ do this. And if the subject being murdered is a citizen of the USA and the gov't doesn't respond with something, then they are implicit.
However, if the subject is not a citizen of the USA, then the USA does not have the legal right to respond (other than to talk trash about it).
I'm not saying the murder was just - it isn't. I'm saying that there's no mental gymnastics, and there's little to limited things the US can respond with.