They few times I've heard Trump refer to his brother Fred, including once in a statement re: Hunter Biden, suggests he may have some compassion for Joe Biden's condition, and the disgust for callousness of the people exploiting that condition for their own aggrandizement.
That’s my take too. Trump is not the animal the media wants us to think he is.
During the debate, I saw a true look of concern on Trump’s face when Joe started going off the rails. Trump could have hammered him and made it worse. I think Trump held back because of his compassion for a fellow human being in trouble.