For all the speculation about why Stalker-kun is such a dumbass, I kind of think the biggest reason is just that he's basically a teen boy and this is his first love/obsession. I don't know how old he actually is (since monster kitsunes live longer), but emotionally he seems to be about 16 year old boy, which is... yeah. Selfish, stupid, shortsighted. Not proud of it but I've been there! Of course then you add in that like others have said, he's an animal first, and a lot of this even becomes understandable from an animal standpoint. You kill the old male and she'll be so overcome with your manliness that it's just a given, rite?