I mean, HRC earned 3+ million more popular votes than Trump in 2016, and Biden is outpolling Trump at this point for 2020, so they at least meet the minimum criteria of viability as candidates. (A concept you introduced to this discussion, not me)
The examples of Trump’s awful actions are too numerous to list. I will say that the thing that finally turned me against him for good was Ukraine-gate. (Not Russia-gate or anything else). Up until that point I remained minimally hopeful he could turn things around.
But anyone who would actively try to recruit a foreign government to interfere in our election, and treat congrasionally-appropriated military foreign aid funds as a personal favor to be granted or revoked at will, is not fit to be president.