In general, it's hard to fault someone who tries to pay as little in taxes as possible as long as it's legal -- and, no matter how rigged (to use a Trumpism) the tax system is for the rich, Trump has at least so far never been convicted of tax fraud.
But what is revolting is that Trump has shown no remorse about, much less any willingness to help, all those people he stiffed. And given what we now about his self-described tactics, it's hard not to suspect that he knew he was going to screw all those people who believed in him from the beginning.
There is a difference between standing right up against a line of what is and what isn't legal -- and going miles beyond a line marking sleazy and slimy business behavior.
Maybe, as Trump boasted in his first debate with Hillary Clinton, such behavior makes him "smart."
But it also makes him a man of deep moral depravity and degeneracy and one unfit for any public trust, much less being president of the United States.