Honestly, your response is why so many people can't resist nasty broadsides at Trump supporters. When someone can't admit that the candidate they support said/did something stupid (and they ALL do that at one point or another), they appear a zealot rather than a thinking person.
Trump saying that McCain is not a hero because he got captured, and that he prefers people who don't get captured, was an insult to every POW. There's no way around that. I've known a few POW's personally , and whatever you think of McCain in particular, the rest of them did not deserve Trump's "I prefer people who don't get captured." Even Trump himself has backed off that statement by saying "they were all heroes whether they were captured or not." He's too egotistical to admit that he regrets saying it, but it's clear in context that he's taking it back.
If he can acknowledge (in his own Trump way) that he was wrong, why can't you?
It was an unwise statement by Mr. Trump...but as you noted...his follow up that "they were all heroes whether they were captured or not" makes it pretty clear that this is about personal animus between himself and Mr. McCain.
I've spoken with and heard lectures from guys who were in the Hanoi Hilton with McCain, and John is undoubtedly a genuine hero. He took beatings for other guys and stayed on when he could have gotten out of there early. That said, I also know a number of people who know him quite well on a personal level here in Arizona...and its a pretty universal opinion that he's a huge jerk with a very bad temper. A complicated man, but just not a nice guy. So like all things in life, relationships and people are complex.
Mr. Trump and Mr. McCain dislike each other and their words make that very clear. Trump should have been careful to keep his criticisms of McCain insular so as not to cast aspersions on other veterans and POW's...because THAT was never his intention nor does it reflect his personal opinion on veterans...but it is, admittedly, an obvious shortcoming of his that he often makes broad brush statements when he really intends to hit a specific target.
That unpleasant tendency to "say something stupid" happens because Trump is unpolished, not a professional politician carefully managing his language to eliminate every misinterpretation or conflation of his comments. He's a businessman and a New Yorker who attacks with unrestrained vigor, sometimes unintentionally painting with a brush that is crass and overly broad. That lack of polish, call it crudeness or whatever you wish, is also evidence of candor and is quite honestly a part of why people are willing to trust him and support him. But clearly, its a two edged sword.
Ultimately, his shortcomings are tied to his strengths (I suppose that is true of everyone). I'd rather have a guy who blurts out crass honesty with a piss poor filter, than a polished guy who's words reveal nothing of his inner thoughts or intentions....which is what 95% of politicians do.
And lets not forget, Donald Trump is wildly popular amongst the troops and with veterans. A recent Stars and Stripes poll showed the troops support him over Hillary be more than a 2-1 margin...lacking any real data, anecdotally, it seems very likely that he is even stronger with older veterans.