Both use the notion of swamps as places of death and fetidness. I wonder where that comes from.
I think there are four sources.
One is that swamps are treacherous. It is, at least in stories, fairly common that a piece of solid-looking land turns out to be anything but, and instead turns out to be some kind of mud or quicksand that sucks you to your grave (yes, I know quicksand doesn't work like that IRL, but we're talking fantasy connotations here).
Another is that swamps are stagnant. Swamp water moves slowly, if at all. That means that things accumulate in it. That's usually life... but it's generally not life that's good for humans. You get dirt in the water, and stagnant pools that are great for breeding mosquitos and other insects that spread disease. Compare to running river water, which is generally considered clean unless polluted by a major city or something.
Related to that: an abundance of life means an abundance of things that die. And since swamp water is usually stagnant, it's usually under-oxygenated, which means things don't decompose as quickly, so you'll find more dead things in swamps. That, IIRC, is the reason most fossils are found in areas that were formerly swamps – dead things get buried in sediment and preserved until the sediment eventually replaces the organic tissues with stone.
And a fourth is that swamps are inaccessible. Even more than hills, it's hard to project force into a swamp. Mounts are unusable. Vision is limited. Hiding is easy. So for people who want to stay away from authority, swamps make good hideouts.