How to deal with accusations from the person I care for?

Tell dad you're sorry for being selfish & wanting to keep him with you here on Earth. Then bring over some papers and tear them up in front of him. Tell him the signed paperwork is now null & void, so God is free to take him anytime He is ready. Be prepared to repeat that maneuver over & over & over again, however, as he will forget you've done it in 10 minutes time. It may calm him down though, and that's the goal with dementia: Keep dad calm & relaxed, whatever it takes. And remember: HE believes you are preventing him from joining his wife in heaven; this is HIS reality. Join it, don't fight it.

Good luck!

I think I would tell my dad that the paperwork actually said I would be there for him until it was his time to join mother in Heaven. When God's ready for him he will go.

I think you will have to understand that this is dementia speaking, not your father. It is not normal thinking, and while you do not tell us about your Dad, or his diagnosis, I would bet there is some dementia involved. You cannot argue with a disease or a condition. It is best to just keep telling your Dad, who clearly is a believer, that his Lord will decide when he can join his beloved wife in heaven, and that you would never try to stop that, that you will miss and love them both forever, but celebrate that they are together again.
There is little you can do to change a demented mind. I am sorry for your Dad's struggle.