Seeds of Despair
Stormriders are described as having seaweed hair, yellow eyes, and angular facial features. They don’t seem like they can pass for human. It is mentioned somewhere that the Stormriders came to Wu by accident/as collateral damage of a reckless ritual by a Jaghut. Like flotsam they probably washed into the world when the metaphorical gates were opened. Just like the giant transforming lizard fish came into the nascent from the Aral Gamelon world when Rhulad opened a portal.
As far as I can see there are two quotes relevant to the Riders’ background in NoN (pages are PS). The first (p.234) is Edgewalker to Kiska, describing the iceberg that is trying to enter Shadow: “It is alien to this realm. It reminds me of the Jaghut, but profoundly alien. They, at least, were not so different. It is said that long ago the Jaghut inadvertently allowed it into this world when they wrought their ice-magic too strongly.” I take the first alien to refer to the fact that it is not from Shadow but the second alien clearly refers to the fact that they are more otherworldly than the Jaghut. The second quote (p.260) can be found when Edgewalker confronts Jhenna and it seems to suggest not only a Jaghut-rider connection but maybe even a alliance: “I bring you sign of your failure, Jhenna. The Riders have been repulsed. No release will come from that avenue this Conjunction.” And not to rely solely on a single source Tay finds a “glowing heart of otherworldly ice” at the core of the riders attack (224) and he calls them “these alien beings” (225). So they indeed seem to be demons (read beings from another world) that ended up here due to some Jaghut accident (could have been a ritual but is doesn’t have to be). But it almost feels like none of them were there of their own will. And the way the Jaghut in the house nearly froze Temper made me feel that she managed to extend her influence outside the house and over them and used them as a diversion and an incredible force.
This idea of the Jaghut being not initially aspected with OP/ice actually sits well with me after DoD, since we got to see all those scenes of Icarias when it was inhabited, and it was full of (pre-Death War) Jaghut, and no ice was seen (but lots of water). Not to mention that their green-skinned-ness would well-suit a water-based evolution from frogs or such.
“why are you killing us?” I think it was a general question. Probably something in the lines of the old Jaghut rituals are fading now and with it the ice of the Stormriders. That or there’s something funky going on at the Stormwall we haven’t heard of as of yet.