I don't give a shit about my grandfather's death
Was your grandfather not important to you?
No, the death is a sad event, but my life is more important.
I understand taking importance in your own life, but you also have to be there for family.   Not care about your grandfather's death is disrespectful to him and rude to the family.
It's not like he was a good guy.
When a family member passes away, it would be polite to offer your condolences. Even if you were not close or if they were rude to you, it shows how noble you are to say a quick passing goodbye.