This is my personal top 5 list, any of which alone is enough for me to not vote for someone and I would've hoped would be disqualifying for most Americans:
1.
Pardoned Blackwater mercenaries convicted of murdering civilians. Whether he did it because Erik Prince raised a bunch of money for him or because it was Biden and Obama who put them in jail or because he believed conservative conspiracies on the case I don't really care. As established in court, they unprovoked killed over a dozen civilians and they were convicted twice based on the clear facts of the case. This is not how Presidential pardon power should be utilized.
2.
Added $8 trillion to the debt. Part of that is COVID, but a lot of it isn't. If you campaign on eliminating the deficit and then spend more than even the worst liberals that's bad. He is the least fiscally responsible president of my lifetime, and while he'll be six feet under before there are any major ramifications my generation is fucked and likely to eat ever-increasing taxes over the next 20-30 years to pay for his giveaway to big business.
3.
2020 Election tomfoolery. It's one thing to lie about fraud and how the election was "stolen" and lose every court case. He has the right to explore all legal options and lose. The issues come from 1) you can't be calling states like Georgia and pressuring elected officials to "find votes" for you 2) you can't be flying in lawmakers from Michigan and ask them not to certify / hand you an election win in their legislature "just because" 3) you can't be pressuring Mike Pence to do unconstitutional stuff while lying to your constituents about his role and responsibility. Forget January 6th for a second... it was the lead up to January 6th where he demonstrated there weren't limits to what he was willing to do or say to try to remain in power. The only thing that stopped him were other Republicans going "nah that's illegal, you shouldn't be asking me to do that."
4.
Much of his inner circle went to prison for campaign crimes or other illegalities. In short, something like 8 of his associates including one of his campaign managers went to jail. His associates did tons of illegal stuff on his behalf. He pardoned all or most of them that stayed loyal, but ones that rolled over (like Cohen) he did not. That is incredibly corrupt.
5.
Hatch Act Violations and Other Corruption. There were something like a dozen Trump officials who committed Hatch act violations, dozens of individual instances of persons using their positions for personal or political profit, de facto money laundering through his properties, etc. This is already a long post so I won't get into all of the nitty gritty but it is well documented how his administration had a very casual relationship with the law and ethics.
These are just some of the reasons I think he's unfit to be President. Any one of these would be enough for me not to vote for someone. The part that's shocking to me is that Republicans could've replaced him with DeSantis, etc. and did no such thing. So now your choices are Trump, Biden, or don't vote. For completely different reasons, I also believe that Biden has quite a bit of "disqualifying" conduct. I do not -- in this case -- actually think there is a "lesser" of two evils because they are both
so bad in different ways that the country is kind of uniquely fucked whoever wins. I wish our choices were people like McCain, Obama, Romney, Haley, Youngkin, DeSantis, Beshear, etc. But they aren't, so I guess I'll just have to sit back and watch the circus.