
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

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 wonder if there's a novel for this manga,love her story a lot!!

there's.. but it's only 5 chapters #-.-)
https://www.novelupdates.com/series/may-i-ask-for-one-final-thing/

I get it, being in a relationship for a long time sometimes feels stale and ordinary that you think to yourself "where has the spark gone?" So when someone new gets your interest you think to yourself this must be it! This is what's missing in your life. But it all ends up the same... with heartache and betrayal all over again... he's better off single and fuck whomever if he wants that honeymoon romance(it's the term for that feeling of excitement in The first phase of the relationship)... he will end up regretting things in the end..

I strongly agree. Some people are not meant for relationships. If its that easy to cheat for him and based on his selfish behaviour he should just be single and fuck around.
I've read quite a few isekai and i can say that i'm looking forward to seeing this story's development!
A child without a loving environment will eventually think it's okay to be bad because no one showed the child kindness and love.. and for her to love and care for him now,there will be a chance he'll grow up a different person!(▰˘◡˘▰)