Hm? The Emperor isn't dead. Zodd helped destroy mist, is all.then the kushan army will know it and griffith will take credit for ridding the kushan threat from their homeland. Griffith is too good of a tactician to let Guts get the credit heh. even though guts technically killed the kushan emperor, Zodd helped, and Griffith will almost certainly get the credit for saving the city.
There are limits to what a good tactician can do. If people see Gutts killing the Emperor, and if people know Griffith was in a whole 'nother place when it happened, there is nothing Griffith can do to change that (short of erasing their memories).