It's interesting that the neuroprotective chemicals also happen to result in a pleasant feeling, as there would have been no evolutionary pressure whatsoever to make a physiological state that's almost exclusively experienced just before death feel like anything in particular. Anything from bliss to agonizing pain to nothing would have been equally viable.

In fact there may be an evolutionary advantage in doing the opposite - flooding the system with stress/fear chemicals at the time of death. Predators would likely prefer to avoid tense, stringy meat reeking of primal panic and feces.

Doubtful, given the amount of large predators that eat carrion and prey on sick and weak individuals in a herd.