I think I might be part of the reason why my grandfather died.
What did you do that you think caused him to die?
I told him to do something that scared him and he died of a heart attack.
I'm sorry for your loss, but you wouldn't have known it would scare him. It's not really your fault, old people are just more fragile to the everyday things. You need to stop blaming yourself because he wouldn't like that, and you need to move forward instead of torturing yourself like this.
I think I'll visit his grave and talk to him.
I think visiting your grandfather's grave and talking with him is a good idea, as it will help you to forgive yourself. It isn't good to hold on to guilt, especially when it's for something that isn't even your fault.