Ok, I think we might have a confusion in terminology. You just alternatingly used the terms "arc" and "act." When I say "act," I mean in the style of a 3-act play. I'm talking the final 1/3 of the story in it's entirety.
The 1st ACT would be story arcs 1-8. The 2nd act is arcs 9-17. The 3rd act is arcs 20-30. (You will notice I left arcs 18 and 19 out. This is because they are in a bit of a strange transition period where they could just as easily fit the theme of the 2nd or the 3rd act, and I'm really not sure how to categorize them. Arcs 18 and 19 have the same degree of horror and death as the arcs of the 2nd act, but they also have something of that beginning air of vindication and pulling back the veil of the 3rd act. So, I really don't know whether to call them the climactic finish to the 2nd act (because arc 17 really does manage to feel like a pretty good cap-stone to the 2nd act as it is) or the point where they're just managing to pull themselves out of the muck and into the 3rd act, but still pretty ugly looking because the climb is only just starting.)
Also, if you only made it up to the 3rd arc, then... yeah, maybe Worm really isn't for you, because it's the 2nd act, arcs 9-18, where it gets REALLY freaking dark. If you were already feeling it having not even finished the 1st act yet, I'd say you're not going to make it through the 2nd act at all.