If Trump was a smart terrible person, says Josh Marshall of Talking Points Memo, his response to Khizr Khan's speech at the DNC would have been, "I grieve for the Khans' loss and I very much respect their opinion and their courage. But I believe the policy I have outlined is necessary for our national security for the following reasons ..." But he chose to say nasty things about the parents of a dead soldier, proving yet again he is a dumb terrible person.
When Khizr Khan and his wife Ghazala appeared at the Democratic convention they attacked and shamed Trump. He no doubt experienced it that way and the chorus of approbation the Khans received from virtually every part of the political spectrum deepened his sense of humiliation and loss of status and standing. As I've noted in so many contexts, the need to assert dominance is at the root of all of Trump's actions. His whole way of understanding the world is one made up of dominators and the dominated. There's no infinite grey middle ground, where most of us live the vast majority of our human relationships. That's why even those who are conspicuously loyal are routinely humiliated in public. In that schema, Trump simply had no choice but to lash out, to rebalance the equation of dominance in his favor. It's an impulse that goes beyond reason or any deliberation. That's what left so many would-be or maybe allies flabbergasted at how or why he would have walked straight into such a buzzsaw of outrage.
For a narcissist like Trump, the rage and emotional disequilibrium of being dominated, humiliated is simply too much to bear. He must lash out. What he said in one of his tweets responding to the Khans is perhaps the most telling. "I was viciously attacked by Mr. Khan at the Democratic Convention. Am I not allowed to respond?" The use of the adverb "viciously" is a good tell that Trump is a narcissist. But setting that aside, most people would know that the answer is "No, you're not." Certainly you're not allowed to respond in the sense of attacking back. Their son died serving the country. You don't get to attack them. Someone with a moral consciousness who is capable to empathy would understand this through a moral prism. A smart terrible person would understand it as a matter of pragmatism. Smart terrible people spend time to understand human behavior, even if the moral dimension of it is invisible to them or a matter of indifference. Just as importantly, they have impulse control.