[mike_hock]

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.

[Liquix]

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.

[hydrok9]

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