Trump has this entitlement thing growing out of every pore of his skin. He sincerely believes Obama is illegitimately occupying the White House simply because Obama is NOT white. Trump wants to correct that.
I also suspect that he only decided to run after his exchange with Obama over the Birther stand he took. Trump believed that if a black man can win the presidency, he should easily grab it as easily as wiping his big ass. It was supposed to be a walk in the park.
There often happens to be some basic similarities between bigots regardless of where on the globe they are located. Trump believed that whites are in the majority and will alone vote him into office. If you dial back to 2006 - 2007 you will find Kikuyu bigots proclaiming that they did not need any other tribe and that as long as they have a turnout of 90% they will run away with the presidency. We know that they got it badly wrong and had to resort to stealing as usual.
Trump as I said is greatly pained by the fact that he is staring one of the biggest electoral losses in history. If things go as they are going and if by some luck he continues to tweet and allow his eyes to veer off the teleprompter, I see him breaking Walter Mondale's record.
A man used to bankruptcies ( and even profiting from them) would swallow it and well exploit it commercially. However there is no such possibility. His brand is already facing a backlash arising from the hate and venom from his own mouth.
Trump is however most pained by the fact that he is losing what a black man got "easily". He is above all losing to a girl. The girl that the black man beat! That is when it gets personal.
Whoever is advising him better pull up his socks. With Obama's current high approval ratings, the candidate attacking him loses votes.
That said, I never believed a madman would run for the highest office in America and be within range of the nuclear bomb. I am told he as already loudly wondered why it is not frequently used, saying "what is it for if it is never used?"