Ah, okay. Nicolas had a rough time in canon, sadly ;A; 

[^X Nicolas by @unionthesalmon – plz reblog from X or the source]

“Armand was a little bitch just because of the whole Nicki thing.” – Armand may have been trying to help Nicki in the ways he knew how. Armand had been a coven master for hundreds of years, dealt with madness from many ages of vampires, maybe this was something that helped in other cases. It could be seen as cruel from our mortal standards, but maybe that was considered a reasonable form of treatment for vampires. 

We only have the account of Nicolas and Armand’s interactions in Eleni’s letters and very little is said. No one ever brings it up again (unless they do in PLROA, which I still haven’t finished), and since we only have the one account, I can’t jump to the conclusion that Armand was definitely torturing Nicki. He can be cruel, but Lestat asked him to take good care of Nicki, and I feel like Armand tried to do the right thing.

“And I agree with you that Nicki was mentally ill” – Some ppl headcanon that he was, and I don’t know what I think about that, but again, maybe Armand was trying to treat the illness and save Nicki! 

“And I freaking loved Nicki, I cried when he died. If Armand hadn’t have done that, would Nicki had lived on? Maybe not, but still.” – If we go by my theory that Armand was trying to help him, maybe Armand’s treatment prolonged Nicki’s life. We just don’t know.

If Armand was really torturing Nicki, I think we would have found out more about it in TVA, or some other book, or Lestat would have confronted Armand about it. 

But whether Armand really antagonized Nicolas to his death or not, Nicolas had enough reason on his own to end badly even before Armand got involved… as Nicolas tells Lestat, it was his intention all along to fail:

“All a misunderstanding, my love, ” he said. Acid on the tongue.
The blood sweat had broken out again, and his eyes glistened as if they
were wet. “It was to hurt others, don’t you see, the violin playing, to
anger them, to secure for me an island where they could not rule.
They would watch my ruin, unable to do anything about it.” I didn’t
answer. I wanted him to go on.

“And when we decided to go to Paris, I thought we would starve in
Paris, that we would go down and down and down. It was what I
wanted, rather than what they wanted, that I, the favored son, should
rise for them. I thought we would go down! We were supposed to go
down.”

Perhaps becoming a vampire was not the cure for that intention/feeling/illness, and it just magnified the self-destruction he already felt ;A;