How so, if the Magistrate is still alive, then he can be exposed. Any chance of that goes out the window with his death. Especially if the one accused of killing everyone is the one who killed the Magistrate, if front of tons of people.
That's only but a chance of him getting caught, and in the meantime he'll keep doing it, especially since the Wano people will put those deaths on Zoro, especially after not committing seppuku or even trying to bringing innocence to his name, instead opting to attack officials and running away afterwards, which only made things worse.
I'll just wait and see and if there's no mention of the Magistrate being alive/being arrested for being the real killer, I'll just claim he's dead.