Is death by fire more painful than death by heart attack, or does the sensation of death depend on a person's faith, where God seizes the soul of a believer with less pain than that of an infidel, regardless of the manner of death?

The difficulty of death varies. Some deaths, such as burning and drowning, are more severe than others. Therefore, Allah, out of His grace, has granted the reward of martyrs to those who die in such ways, due to the intensity of the pain. Death by burning is more severe than a heart attack, because burning is a form of martyrdom, while a heart attack is a sudden death and a rest for the believer. When the soul is being extracted, the soul of the believer exits easily and gently, whereas the soul of the disbeliever disperses within the body and is extracted with severity. The alleviation of suffering for the believer when the soul departs does not contradict the intensity of the death throes or the manner of death. Indeed, death was severe for the Prophet, peace be upon him. The severity of death for a believer is an increase in his good deeds or an expiation for his sins.