well, we are talking about a 14 years old kid, he is so fair with them. and he may never think about keep an enemy alive with him at home as a pet, it's a big risk :D you don't think?
:P There are plenty of reasons to spare at least one Doll~let me explain:
1.In video game history, Dolls is maybe the most pathetic zako with low health and incompetence skill for killing. That's why they always team up in battle - because they are just normal human lack of real ideal(their crime is only work for poachers). When the hero kill them all except one, and she seems no other way to survive, her will is easily bend by the hero's cruelty action.
2.When our young hero is overpowered them, an individual Doll really can't do anything about it. Keeping a Doll alive for further useful information and some extra bonus is less risk than other zakos with firm faith.
3.She is just a poacher XD, not some god damn terrorist! Even though she tried to ambush the hero, her sisters had paid their crimes really well. Just let our hero gives her a chance~ Torturing henchman who doesn't want to fight back is not the hero's way.
4.You know... The hero spread his sperms into these Dolls, maybe it will make a new life... Child is not as guilty as their mother!
5.Maybe there is a Doll just like a little-brother-like young man? Please let our hero gives it a try!!
Keeping a Doll alive is not the decision that the 14 years old hero will regret, at least he can send the last one to cops!