Each character in the Straw Hat crew has its own meaning in the group, but also has relationships with the outside world and qualities that everyone needs. One case was that of Sanji, a member of the Vinsmoke family who felt deeply involved in Big Mom and his crew and caused Luffy's invasion of Whole Cake Island.
But there are also other members of the Mugiwara who have piqued the interest of the strongest pirates in the world of ONE PIECE. While Sanji continues to get the queen's attention For the technology of the Vinsmoke, Big Mom confirms that the emperors want another member of the group of protagonists on their side.
On the Finale of Chapter 999 of ONE PIECE It is confirmed that Pudding is currently unable to read poneglyphs, and therefore the ability to achieve Laugh Tale for Big Mom and Kaido is tied to a different character. Obviously, this character is the Straw Hat Crew archaeologist, Nico Robin, the last one in the world who could read the ancient language carved on the poneglyphs.
The need is not to kill them during the battle that is taking place in Onigashima for Then force them to read the engravings on the stones that are in the possession of the emperors. But of course Luffy doesn't allow this to happen so easily.