I probably should have left this alone.  But for some reason, I just couldn't help myself.

I think there's some confusion about the passive voice here. "unfold" is actually in the active voice in this case. By changing it to "was perpetrated by", I think you have actually changed it from the active voice to the passive voice.

— Joel Swagman (@JoelswagmanJoel) January 30, 2022

You could say "British soldiers perpetrated Bloody Sunday" if you want to avoid the passive voice.

— Joel Swagman (@JoelswagmanJoel) January 30, 2022

I just googled "middle voice" now. It looks interesting. But I wonder... the examples they use seem to be when the verb has both a transitive and intransitive form. Would this work for "unfold"? https://t.co/Ghl8g0jnlG. pic.twitter.com/oaTChqRExG

— Joel Swagman (@JoelswagmanJoel) January 30, 2022

From my (admittedly) 2 minutes of research, I get the impression that middle voice would refer to cases in which the verb has the option of taking several different voices. I think with "unfold", there's just the one form, no?

— Joel Swagman (@JoelswagmanJoel) January 30, 2022

You mean in the "unfolded" sentence or in the "was perpetrated by" sentence?

— Joel Swagman (@JoelswagmanJoel) January 31, 2022

Yes, that's my understanding of the sentence. "horror" is of course an abstract noun, but English has the ability to nominalize just about any concept. And then those abstract nouns can syntactically fill out the slots of either active or passive sentences

— Joel Swagman (@JoelswagmanJoel) January 31, 2022