PN,
Humanity is a fundemental of LOAC. You seem to disagree. Would you say that there was no difference morally from using a clinical weapon e.g. bullet to temple and one that caused a painful death e.g. lowering you into a meat grinder? I would say that the difference was clear. So does the law.
Examples deliberately chosen to be left and right of arc.
The point stands. Both in practice and in law one weapon system is indeed more, or less, humane than the next.
I think your point is that to kill is to kill, which is valid to a degree.