Kizaru is his bodyguard so it’s his job to protect him from luffy at the very least
This is only half of the story, Kizaru and Saturn went to Egghead for Vegapunk, not for Luffy.
Kizaru was not sent to protect Saturn from Luffy, he was sent to escort Saturn and kill Vegapunk.
The problem is that Luffy's ship appeared on their way to Egghead and this is the Egghead incident (read the pages after this one).
The Egghead incident is Nika being in Egghead and paralyizing an admiral so that he does not accomplish his mission.
This incident will escalate and its resolution will shock the entire world: Saturn interferes, Nika defeats Saturn, a celestial dragon.