Hello sweetie,
You sound really scared and worried and your grandfather is lucky to have you. Stopping taking insulin will cause you to die-eventually but it would be slow and not nice. I would strongly NOT recommend this course of action, although I can understand your grandfather's comments of wanting to die. Poor chap.. I think the best you can do dear, is sit with him, hold his hand, reassure him and just by being there will help. Nature will take its course and if he is having a lot of heart attacks, that is not so good and he may pass on soon anyway. In New Zealand we have a saying: Kia kaha. It means "stay strong". So give your grandfather as much love and support as you can and look after yourself too.
All the best,
Lisa