Generally I'd agree about people being entitled to voice political opinions I disagree with. But no, in this case he isn't a legitimate candidate and people are not entitled to support him.
He's a fascist who has already incited an insurrection to overthrow the democratic process when it didn't go his way, and has made no secret of his willingness to do so again.
You can argue about whatever you like within the scope of what a democratically elected government might do, with the standing proviso that if people don't like it they can vote them out. Argue about tax rates, immigration policy, abortion, whatever. Sure, make an argument that Trump is right about these things and if enough people agree, those of us who hate him will just have to suck it up.
But once you start denying the principles of democracy itself, and promoting someone to be leader who refuses to acknowledge them, who intends to impose pure authoritarianism instead, then you're having a different argument.
You can't have a legitimate candidate in a race who doesn't acknowledge the rules of the race.