This is a great post (and distinction). The vast majority of people do experience that upsurge in adrenalin and related chemicals while hunting, we're designed that way.
There is a very small subset of people whose brains are different to the rest of us. The popular phrase is "cold-blooded killer." However, it's clear that some people do not react emotionally/with adrenalin to stimuli that would have the rest of us fighting, fleeing, fainting or out of our minds with panic.
I also agree that even one of the people whose brains are more hard-wired to allow them to proceed calmly in terrible situations would likely experience some degree of agitation/adrenalin during a murder, especially if it were their first. Killers like DeAngelo (EARONS/GoldenStateKiller) are rare. He was able to munch on his victim's food in the rape cases, and, perhaps, in at least one of his serial murders. I believe he worked his way up from that but had the ability to rape/murder and then appear calm to his family and, at one point, his LE co-workers. Rare.
I don't think this case is that kind, though. If you are right, then either this killer is a loner or was able to calm himself down in a steely manner very soon after the killings.