I'm struggling not to repeat myself, but suppose I wrote a book with the first chapter titled "Inside mikeash's Mind" that purported to diagnose you with the propensity to make irrational decisions, and proceeded to document this inability by referring to a former employee who resigned after a year rather than be fired and selectively quoting something written about you by a non-associate...and nothing else. It's hard to see how that would not be a slander[1] on you, regardless of whether you've done more good or ill in your life.

[1] Not in the legal sense, of course, since it's written, not spoken.

If I had had a long public life filled with actions whose only explanation are malice or irrationality, I don't think it would be slander, no, even if the argument wasn't very well constructed. That doesn't make it a good argument, I just don't think it qualifies as slander. (Or libel.) For one thing, slander or libel in the US has to be actually false.

The flawed assumption in your argument is that malice and irrationality are the only possible explanations for his actions.

I'll grant assumption, but flawed? What else is there? I recognize that there are some people who will explain his actions as "brilliant, wonderful, generous, pure of heart, etc." but I have no qualms simply calling them wrong.