Making something abnormal seem normal?

Since you explicitly mention that you don't know if it was a verb or an adjective I think you will fare best with makes the scene seem adjective or describes the scene as adjective. The reason is that the effect of a scene seeming normal in any given context is likely conveyed by the persons reacting unexcited and without the disgust the reader feels.

Just picking a few adjectives from the synonym list of commonplace here, ordinary, unremarkable, conventional, typical, everyday...

The writer makes the scene seem ordinary.

The writer's describes the scene as commonplace.

This disgusting scene is skilfully made to seem unremarkable.

This is on the assumption that the scene's sense of normalcy is brought forth by it being an everyday thing for the participating persons. If you are trying to describe a writer that wants the reader to think that the scene is normal in our world that would be different there is more likely a verb out there that fits that description because it's an Inception level fewer.

neutralize

I'm sorry, but the definitions I read don't support this idea. Nevertheless, I think it works in the context. The context involves some sort of tension which can be neutralized -- the tension gets removed by the matter-of-fact treatment the author gives the material.

clinical

Dictionary.com gives

"3. extremely objective and realistic; dispassionately analytic; unemotionally critical"

The writer's description of the scene is clinical.

The writer neutralizes the scene.

This disgusting scene is dissected by the writer.

Dissected means the writer took apart the offensive details, removing the emotional effect.

Maybe detached? This has a connotation of emotional coldness or numbness

adj. impartial or objective; disinterested; unbiased; not involved or concerned; aloof.

It fits best with your first sentence:

The writer's description of the scene is detached.

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I might also suggest something in the vein of nonchalant, which fits a bit better with more of your sentences.

adj. relaxed and calm in a way that shows that you do not care or are not worried about anything

The writer's decription of the scene is nonchalant.

The writer describes the scene nonchalantly.

You should consider trivial/trivialize (UK trivialise).

The writer's description of the scene is trivial.

The writer trivializes the scene.

This disgusting scene is trivialized by the writer.

M-W:

trivial adjective

1 : commonplace, ordinary

Compared to her problems, our problems seem trivial.

trivialize transitive verb trivialized trivializing

: to make trivial : reduce to triviality

The news story trivialized the problem.

How about regular / regularize?

From Dictionary.com:

regular: usual; normal

regularize: to make regular

The OP's examples:

The writer's description of the scene is regular.

The writer regularizes the scene.

This disgusting scene is regularized by the writer.

dispassionate

: not influenced by strong feeling
especially : not affected by personal or emotional involvement

https://www.merriam-webster.com/dictionary/dispassionate

unimpassioned

having or showing no emotion or intensity

Oxford Dictionary of English

Soft-pedal seems to work well in the provided context.

To make less emphatic or obvious; play down:

soft-pedal a potentially explosive issue

[American Heritage Dictionary]