thanks for raising this very important topic. i'm not a military historian, but I am a historian, and one of the fascinating things about history for me, is observing what seemed "normal" at one time or other in history, which might be incomprehensible by today's standards. The Napoleonic era abounds with such things - sitting on the operating table while a bloke sawed your leg off was just one. I agree with you that the concept of a "war crime" as such cannot have existed at the time. The rubrics "ungentlemanly conduct" or "against the rules of war", which were used at the time, have quite different (albeit nuanced) connotations.
And yes, political correctness and viewing the past through the eyes of 2003 are real problems if one wishes really to get to grips with what people thought at the time. I find reading novels of the period is really useful. There you get the "horse's mouth" view. Eg, Jane Austen (who led a very sheltered life) knew perfectly well what was involved in "capturing prizes" at sea, but there is no flicker of denunciation, in fact she admires her hero's (in Persuasion) having become rich this way. Frankly, I think a common view at the period would have been along the lines of "you can't make an omelette without breaking eggs."
Something that always strikes me, especiaally when I read memoirs of ordinary soldiers and seamen, is what a very robust view of life people seem to have had. In comparison we are very sheltered indeed.