Xykon wants to cast a ritual to control the Snarl, correct. Redcloak misled him, since the ritual will really allow his god (the Dark One) to teleport the Snarl into the realm of the gods: this will be used to force the gods to accede to his demands to improve the lot of all goblinoids, which pre-Gobbotopia basically boiled down to EXP-fodder. The problem with the ritual, though, is that it requires an intact, accessible gate. The only one they sat on for any amount of time, Dorukan's, was shielded so they couldn't access it without someone who was Good. With all the other gates now destroyed, Serini's gate is their last shot at casting the ritual.
Hel, on the other hand, does not want to see the Snarl released. She wants to preemptively destroy the world to reseal the Snarl. This will also kill all mortals on it, and since the dwarves will all die a dishonorable death, all of their souls will automatically default to her. She can then use this combined power to gain a commanding position among the northern pantheon recreation of the new world, and one in which the "trick" Loki and Thor played on her (in which she only gets the souls of dwarves who died dishonorably in this world) has expired.
As for the Snarl, no one knows. We know there's a world, but we don't know how it's related to the Snarl. We know the Snarl is also still on the other side, because it's already ripped through one of rifts left behind by a broken gate. Other than that, it's one of the big last-act mysteries.