I am now quite certain he'd been positively insane when he wrote it

[kttlt]

This is my translation of a diary entry by Tolstoy. I wanted to share this piece with an English speaker, but wasn't able to find an English translation online. I understand that you probably can't comment on the translation quality, so I just want to know if the text reads well in English, and if there's anything I could improve.

"I've been reading Nietzsche's Thus Spoke Zarathustra, alongside his sister's notes on his writing process, and I am now quite certain he'd been positively insane when he wrote it, and not metaphorically, but in the most literal sense of the word: incoherence, jumping from one fragmented thought to another by contrast or consonance alone, comparing without establishing what is being compared, all that fused by a single obsession — idée fixe that in denouncing all the highest principles in which human life and reason are grounded, he manifested his super-human genius. What must a society be, for such a madman — and an evil one at that, — to be considered a teacher?"

I'm particularly unsure about the part of the first sentence following the colon, and whether I used the past perfect correctly.

 

[emsr2d2]

This is my translation of a diary entry by Tolstoy. I wanted to share this piece with an English speaker, but wasn't able to find an English translation online. I understand that you probably can't comment on the translation quality, so I just want to know if the text reads well in English, and if there's anything I could improve.

"I've been reading Nietzsche's Thus Spoke Zarathustra, alongside his sister's notes on his writing process, and I am now quite certain he'd been he was positively insane when he wrote it, and not metaphorically, but in the most literal sense of the word: incoherence, jumping from one fragmented thought to another by contrast or consonance alone, comparing without establishing what is being compared, all that fused by a single obsession — une idée fixe - that in denouncing all the highest principles in which human life and reason are grounded, he manifested his super-human genius. What must a society be no comma here for such a madman — and an evil one at that no comma here — to be considered a teacher?"

I'm particularly unsure about the part of the first sentence following the colon, and whether I used the past perfect correctly.

See above. As you say, we can't comment on the actual translation but your piece is very well-written.
My only other minor gripe is that you opened with "I've" but then used "I am" later in the same sentence. I suggest you choose either contracted or uncontracted forms of verbs.