Your story reminds to one of a famous stand-up comedian in the netherlands:
He also have very serious parts in his show, and once he told he once went to an theater for an performance and met his parents on the road having the same destignation. They didn’t noticed him. They were so very happy and luckaly together. The same moment he wished they would be hit by a train that would kill them instantly.
I was kind of shocked hearing it. But before he explained I understoot. Why? because he loved them! He did not want one of his parents to miss the other, or to suffer those sometimes nasty last days others have. Or see the other suffer.
Earlier this year I’ve had some ackward deaths within my family. I’m very sorry these things just happen, also among the beloved people in your family.
Death is one thing you can be sure of in live. And as you are 17, you probably realize they did’nt want to live forever on this earth. I hope you may share some better days with your granddad, but if not then hopefully above words may help you see at least some positive elements in this sad event.