Fascism is a political ideology and system (sort of) that requires a whole lot of people to keep it in place. A "lone President" assassinating his political rivals in a single act is just a regular old dictator move. No fascism required.

Not endorsing the view, but that's how some people think. It would actually be incredibly interesting if he decided to commit political suicide by having Trump killed and spurring congress into amending the constitution to explicitly revoke the immunity. If presidents start murdering presidential candidates, we might be able to get together and pass a constitutional amendment. :)