You dont understand how evolution works. It's all about passing your genes to to next generation. This means all the traits an organism has before it mates/passes on genetic material are the only ones that matter. Some kind of euphoria or painlessness while dying would have no reason to be passed down the evolutionary chain. Because the ones dying, are not the ones mating. Trust me, dying sucks, unless you get crushed by a 100-ton freight container, it's gonna freakin' hurt.