This strikes me as a particularly good way to run a game for kids, and gets rid of 99.9% of morality issues. Give the kids a choice of star metal weapons and then play the game as usual. Instead of describing damage when the kids hit, say that the Serpent Man blocks your sword stroke but also starts to flicker and glow as a whirling portal opens above his head. If the kid "kills" a Serpent Man just say "With a final mighty blow you banish the Serpent Man- he hisssssess in frustration as he is dragged into the Abyss!"
Maybe don't even use a dmg roll, just use a 3 hit system.
- On the first hit the illusion is destroyed and the scaled Serpent Man is revealed in his true form.
- On the second hit the Serpent Man starts to glow and flicker as a portal opens above his head.
- On the third successful hit the Serpent Man is banished.
Serpent Men make a good foe for another reason- They are master spies and skilled in illusion magic. This is great because Serpent Men become a much more diverse foe then say Orcs for instance, and they could reasonably pop up just about anywhere. A troupe of Orc warriors would stand out like a sore thumb in a market place- but Serpent Man would fit in no problem.
Plus snake people are just really obviously bad guys and there is lots of great reptilian imagery you can throw in along with the pseudo-Egyptian/Babylonian thing Stygia already has going on. Just don't take it too far- Wrath Amon's snake slippers are truly horrible.