The shooter wasn't a strong Republican or Democrat. Based on what little is currently known, he was more like a mentally disturbed incel crying out for attention.

Anyway, even if Trump's followers wouldn't have started a second civil war, they'd have doubled down on Christian nationalism, deeming Trump a messianic figure, and more competent demagogues like Vance would step in to lead the movement.

Of course, I don't have a crystal ball, but this seems to be the main reason not to advocate for Trump's assassination (aside from a commitment to human rights). The problem is there's a justifiable fear of how Trumpian Republicans would react. The truth of the shooter's motive might not matter as much as the conspiracy theory that would spread. Biden could easily be blamed even if Biden would have nothing to do with it. Republicans already live in their own world, under Fox News etc.