The basis of the problem is a generalized cluelessness...
and the usual victim blaming psychology.
Hey, saying "He knew what he was getting into" is not uncommon among vets/career military. It's not really meant to be victim blaming or insensitive. Kelly, as a career Marine, has experienced both sides of that. Hell everybody is a "volunteer" these days. But it is "tough guy" talk.
The problem is that civilians don't think that way. To intrude that kind of "tough guy" talk into the grief of a wife/mother/family member is despicably insensitive. Once again, the problem is that people who lack empathy cannot alter their approach to fit the people they are talking to. Trump has demonstrated this every time he tries to act like he gives a shit. He cannot be genuine in those situations because he has no geniune feelings for anybody or anything outside of his own personal meat sack. Apparently Kelly shared with Trump and Trump thought it sounded good. As usual, he was out of line and has no clue why.