Writers become Gods when creating their characters, but this privilege can surely only be allowed if the characters are rounded and not just mannequins on display, or meat-bags that get taken out in some torture-porn drivel.
Many writers find it useful to use philosophy as an aid to crutch characters.
This seems to be quite a useful tool.
The killer was obsessed with nothing but survival of the fittest – as where the victim is just crazy about love.
These themes can be used quite well. How about, the killer who just “loves to love”. The crazy stalker perhaps. Or a mad fan?
As in the “man murders wife” example, “I love you to death my dear” and “I will murder you if I can’t have you”?
Another perspective – the victim is obsessed with survival of the fittest, but still becomes the victim because he or she let their guard down.