If you’re going with speed over armour, then you should probably go with the scavenger route, making them more like rats, rather than a rare creature. And even if you want to pose a challenge to ranged characters, I’m not sure the feign death thing works like you imagine it would. At best it gets in one good hit, then the character shoots it point blank in the face, and makes a mental note to avoid corpses next time. If you really want, you could just use the flag that skeletons and smokers have that makes them hard to hit with ranged weapons. It’s a long, thin creature so it wouldn’t be that hard to justify.
If you’re committed, you could implement both versions. Have small scavenger snakes that spawn around blood splatters and rotten meat. Have them be relatively peaceful unless the player is bleeding, in which case they turn hostile. I’m not sure how that would be implemented, though, you’d need to add an extra flag to scent, maybe. Maybe this common variant has the blood spit and hard to hit flag, but not much else. Giant blood snakes would be rare, found largely within their blood caves. You could go whole hog on their armour and health, with the exception of the hard to hit flag because they’re huge. They’d probably hit slower, though.
