I understand that, but the prosecutor would have to prove that Chauvin decided he was going to kill Floyd, rather than that he was heartless and ignored the pleas of a man in distress. The latter is in the bag, done, slam dunk. The former is much harder to prove. If the AG thinks he can prove first, he can always add the charge, but it's just good sense to do the initial charge on the slam dunk offense, especially when they needed to get an arrest and charge on the books to calm the rioters.