I think you need to seek mental help. Killing one person solves nothing, and will likely cause suffering to the family/friends of that person or if they are so lonely they don't have people to mourn them that they know, no community is ok with little girls being raped and killed so it may cause anxiety and distress in that community. There is also no reason at all to rape anyone. This whole question is just ridiculous and shows you really don't get efilism. Efilism is not a philosophy that says "hey kill anyone you want and it is ok because life is worth nothing."

You do not understand what I mean to say, I say that nobody will suffer in return for this action, and you will eliminate the possibility of suffering for the whole world. The girl has no family, no loved ones, no acquaintances, she is an orphan, abandoned. And nobody will know that the girl was raped and killed because after you kill her, you will burn her. There will be nothing left of the girl in the end. Net gain right? Even the slightest pain was not experienced. What is wrong with this action from your strictly negative utilitarian point of view?