He can't help it

Jennifer Rubin in the Washington Post:

...The compulsion to attribute intentionality to Trump’s behavior leads to ludicrous explanations. Pundits, for example, too often claim that Trump tries to scare suburban women, a large and critical part of the electorate, so that he can pump up his base — White males who have never abandoned him. This...is almost certainly false. 

Trump does what he does because he cannot help himself. Does anyone really believe that he looks at polls objectively? It seems he cannot plan a day ahead — let alone a month ahead. He certainly cannot control the impulse to insult and degrade others. 

His narcissism and lack of conscience — not calculation — lead him to do things that are self-destructive. There was no benefit for Trump in refusing to denounce white nationalists at Tuesday’s debate. That moment was about Trump refusing to be told to denounce racists...

See also Proud Boys—and Craven Republicans—Stand By Trump.