I wouldn't care if my grandma died
Are you serious? Why wouldn't you care if your grandma died?
I am not close to my grandma
I understand where you're coming from, but it's not right to say that you wouldn't care if your grandma died. Even if you aren't close to her, other people are, and I'm sure they'll be very upset. Maybe you should try to build a relationship with your grandma, as it could be very rewarding for both of you.
Nah, I'm a terrible grandson.
You might be now, but you can always decide to be a better grandson. I am sure she likes you just for being you and it is good to find time for your grandma.