Of course a decent person can support Trump, if by decent you mean hard working, honest, responsible, etc.
This isn't about the decency or not of Trump supporters. It's about indirectly supporting Trump and thereby increasing the chances he'll be elected.
Altman's and YC's non-disavowal of Thiel supports that outcome. Altman's support is a layer removed from writing a check for Trump, but it's still support.
If Altman believes (as he seems to) that a Trump presidency would be very bad for America, it's hard to see why he would not specifically repudiate Thiel's actions. That's different from repudiating a friendship.
If you had a friend who used their considerable resources to publicly and materially support what you believed to be a truly bad outcome for America, would you not consider saying, "I can't go there with you, and I can't let my company's prestige support you while you do."?
He's NOT indirectly supporting Trump though. You shouldn't HAVE to fire everyone who disagrees with you politically in order to convince "ridgeguy" that you support your own views.
If Altman is comfortable with the idea that his actions are encouraging people to comfortably support Donald Trump, then I'm comfortable with him continuing to endorse Peter Thiel.
You do concede that people (including Thiel) can be decent and support Trump? I know plenty of good people who support him and for good reasons.