The snap kill is mostly a side effect. The true purpose of the snap is for blood to engorge Galactus' penis so he can fire off a waterslide of muck at the inside of his armor. He then uses Palpatine's robes to clean it up since they were conveniently placed near him.
Honestly Galactus doesn't even know Palpatine is there. He just had to beat one down that instant.