i'm assuming: "why would i curse someone to be like me"
he views human's as "lucky" because of their lifespan but they don't understand how lucky they are until it's all gone.
also, there is always a common theme with vampires about having unquenchable thirst so "why would i want to give someone else this curse"
Also looking how Grandpa Butler already had a plan to keep 2 things hostage, The Girl & their Casket, I'd say he hasn't changed from 30(?) years ago. He'd be scary as a vampire cause thirst wouldn't be a hurdle to his immortality. Imagine how many humans would die for blood, if butler gramps (or his underlings) dont Care for human life?
I don't get it as to why Big master won't let Pop's be immortal, he served them for 30 years, he provided them with a place,money,all lavish lifestyle etc.... is it because big master envies humans that they can die? Or big master knows the feeling of not dying? I really wanna know his reason