In all my time building--I never thought I'd spend my time brainstorming ideas by trying to analyze whether Sculk is Parasitic (attacks living creatures) or Saprotrophic (feeds from dead).
I want to revitalize an ancient city--but in a fantasy way. And my brain works best when brainstorming from fact/reality. Taking something real and pushing it to extremes or explaining its transformation with minecraft mechanics/logic. And that's difficult to apply to the mysterious Sculk.
Sculk has been interpreted in so many ways--all stemming from its main elements. Firstly, the sculk/moss/plant/mushroom??? Itself. Secondly the bones. And finally the Souls present in the Warden and Shrieker.
Which lead me to trying to figure out the first, what in the world IS sculk?? Leading me down a brittanica/wikipedia rabbit hole of the fungus kingdom.
So actually: is Sculk Parasitic--attacking and or feeding off the living (not always to kill), or Saprotrophic--coming from greek meanings rotten and nourishment (feeding off the dead).
I think (again my opinion/observations) Sculk is both. As the player exists in the current minecraft version (I say this because of the advance of the fungi life in the nether and technically speaking we saw the pigmen evolve) Sculk is in the middle of evolving.
By all natural means, you'd assume--based of mechanics--Sculk is Saprotrophic. When a creature dies near it, the Sculk spreads. However--thats not necessarily what occurs either. The sculk only spreads near a catalyst, which can be placed anywhere, and will flat out "eat" the blocks around it to turn into sculk. They're gone.
But that speculative at best. The Warden also makes a good case, being the one that drops the catalysts in the first place. It attacks--searches for anything to kill--and if done near a catalyst, it will spread the sculk. Yet that's also happenstance, and Minecraft lore would lead us to believe the warden is man-made.
What cements Sculk's evolution to me, is the sensor and shrieker's role. If sculk was truly Saprotrophic, why does it have scouts? Things that alert and call forth an attack on any living thing it can posses? Which admittedly makes it more of an ant colony than fungi... but I believe that still points towards a parasitic relationship. A highly evolved one at that.
Not only does Sculk feed on the living, it's evolved far enough from it's Saprotrophic origin, that it has its own offensive system. Sculk can't do anything without its own ecosystem.
The sensors scout out new creatures to feed, alerting the shriekers. The shriekers summon the Warden. And the Warden either spreads the sculk further by killing near a catalyst--or dying, allowing the chance for a new catalyst to spread out the sculk even further next time.
I have suspicions about sculk being actually that thought out, but even so, it's a highly compelling ecosystem of its own.
TLDR; I think Sculk used to be Saprotrophic, and in the age of the player, it's evolving into a Parasitic Hive.