I speculate that the outcome would have been different. So yeah maybe he's a straight shooter, but he might not aim as carefully when it's one of his own in the sights.
Just consider that Trump has been using Mueller, the FBI and all of law enforcement as a punching bag for the last two and a half years.
I am not so sure that Mueller is so in love with Trump.
The man has sullied the very concept of law and order, and let's not forget...Trump IS an organized crime boss. Maybe not Cosa Nostra Italian style, but he's a mobbed up boss, and Mueller made his bones taking down the Capo di Tutti Capi.