Is death during sleep a mercy from Allah in that one does not feel the throes of death, or a retribution because one does not utter the Shahada?

All deceased individuals experience the throes of death, varying in intensity, and not everyone who dies in their sleep is deprived of martyrdom. If a Muslim recites the remembrances before sleep, they are under Allah's protection, and if they die, they die upon the natural disposition (fitra), as the Prophet, peace and blessings be upon him, said: "When you go to bed, perform ablution as you would for prayer, then lie down on your right side and say: 'O Allah, I have surrendered my face to You, entrusted my affair to You, and committed my back to You, out of hope and fear of You. There is no refuge or escape from You except to You. I have believed in Your Book which You sent down and Your Prophet whom You sent.' Make these your last words. If you die that night, you will die upon the natural disposition (fitra)."