Sería "deberías haberte marchado antes"

you can say "deberias haberte marchado antes" and that would sound formal but if you want to be agressive you can say "tendrias que haberte ido antes" and that would sound as if you wanted that person to leave befor than what he/she did

The translation in spanish of the verb "to leave" can be : marchar, dejar, salir..that is: to march or to walk away, to leave something, or to get out of somewhere. So, " you should have left before" could mean in spanish: "deberias haberte marchado antes", "deberias haber salido antes", or, "deberias haber dejado antes"