Reactionaries, basically. All of the players thought the demon was bad and weren't interested in the details of Neo Hell beyond poking holes in his plan (which the demon was happy to explain at great length and justify his reasoning; even told them the plan wasn't popular with Hell and he was basically rebelling against the Devil to do it). The reason they jumped in, specifically, is that one of the players is a martial artist who learned his abilities in the faerie realm. He has a sixth sense that allows him to detect "corruption". Note, not 'evil', corruption according to his martial philosophy.
The demon registered as corrupt (not because it was a demon, specifically, but because it was partly an entity from another plane inhabiting the same physical body as a mortal which is considered a corruption in the martial artist's teachings) so he attacked him. When he attacked he got off a surprise attack (which mostly destroyed the demon's right arm and lit him on fire to boot). The demon then tried to flee (3hd to teleport anywhere on the planet) but one of the other PCs attacked the demon as well in the first declared round of combat (knowing full well, since he declared after the demon, that the demon intended to flee and not try to fight them). So one PC shot the demon (5x6) which dropped his teleport (3hd lost 1w to become 2hd) at which point the PC who started the slaughter hit with his 4x9, doing enough damage to fill the body with killing damage. And then the body burned up, leaving just a head. Which the 1st PC burnt. And then they scattered the ashes to the wind and had the moon base computer teleport them away.
Some of the PCs were mad at the martial artist, not because he killed the demon, but because they said he'd killed the mortal host too and the demon was just going to come back again.
One PC, the one with B-Level clearance, has a mortal objection to the whole thing (he tried to stop the murder but failed) and the only reason he hasn't revealed the identity of the murderer to the Arcane Underground is because he has a Loyalty to "Family" and the PC who murdered the demon is his cousin (in game).
Thing is, I actually applaude the murderer in this case because he, as a PC, has picked a morality and is actually following through on his choices and is fully accepting of the fall out of his actions. Most of the other PCs just sort of wander around, trying to avoid making choices whenever possible or trying to find the "best" solution when one doesn't really exist.