Are you just saying this because of Starship Troopers? He explored a lot of ideas through fiction but I don’t think you can call him a supporter of fascism because of that one book, unless you know something else about his personal life. Of all his books I’ve read I think the only real ideological through line is “horny old guy objectifies women and that’s OK”.

Like Stranger in a Strange Land has Space Jesus starting a revolutionary free love commune, I don’t think that’s really in line with fascism.