User blog comment:Fliu/Has Bege Gone Soft?/@comment-53539-20180510175408

It is possible his reputation proceeded him and he always was quite soft.

ITs also notable he has become a father so has taken up a parent role... being a parent, he will know what its like to kill "someone's child".

Sanji likely did whats for the best, as the complete collapse of a Yonkou's territory right now is not a good idea even if that was not the intention. It would leave a gap in the New World that would lead to fighting. The New World already had this with Whitebeard, and now a great "evil" called "Blackbeard" now holds his domain. As such, by removing Big Mom, there is a reason for a greater evil to replace it. In addition her entire family, the races within her domain, all are at risk should she fall.

The mission was also never about killing her, but rescuing Sanji, Bege was the one who wanted to kill her but he had to cooperate with Luffy. Luffy would have been the hinder in any place if Luffy were to go against Bege, including if one of his crew did so. In a alliance in One Piece, be it temporary or long term, both parties have to be both aware of betrayed and willing to par take in the actions. If just one party decides to act different then things are impacted in turn. Luffy is not a killer, he just wants to beat up the other pirates and move on.