I have sat with dying loved ones, both family and friends, and hoped that it would be quick.
Look at how you feel about yourself. Do you want protracted illness or a quick, quiet ending? If you feel that you want the latter for yourself, I do not think that it is so awful to pray for that for your father.
I think that most people want to go before it gets too awful and want that for their loved ones. That's not to knock anyone who clings onto every last drop of life. If you can do that, then go for it.
You say you pray. I infer from that that you are religious, but I don't know which religion you follow and it is none of my business. My answer will, however, be based on the fact that you pray.
However, in all the monotheistic religions I have come across, God is accepted as all-powerful and having the final say on life and death. If you believe that and you pray, you are not telling him what to do, you are telling him what you want him to do. It is down to him to decide what to do. If you believe in God, then surely you believe that he will do what in his judgement is best having regard to all he knows, including your prayers, not what in your judgement is best, therefore it is not wrong to tell him what is in your heart.
I do not know how this might be different if you follow a polytheistic religion.
Remember, praying for someone to die before their life becomes too awful for them to want to live it is not the same as hastening their death. You can look after your father to the best of your ability whilst leaving the final decision to God.
As for me, I do not want Dad to live once he ceases to have quality of life. Once he can no longer enjoy a magazine or his old photos or the radio or his favourite food or an old film or slowly pottering down to the shop, then I will not want him to linger. But I am looking after him. I make sure that when his time comes he will not have died of neglect or for want of being shown love and care. From the sound of it, you are doing the same for your father.
Praying that someone will die when their future is bleak is not the same as killing them. It is putting the decision in God's hands while you get on with helping them live with your care and love until God takes them.
