Feyre is the monster the IC fear Nesta to be (WaR, SF, HOFAS spoilers)

The IC feared that Nesta, in a fit of rage, would lose control of her magic and hurt someone. They didn't want to leave her alone with Feyre because of it. They felt like she needed to be restrained because of it. Yet it was Feyre - not Nesta - who threw a temper tantrum, lost control of her magic, and burned the LoA.

The IC didn't want to tell Nesta the truth that she could Make magical weapons because they worried that she would Make something to spite them. They thought that she was plotting against them, that she was a threat to their court. Yet it was Feyre - not Nesta - who plotted against and destroyed an entire court out of spite.

The IC judged Nesta for being prejudiced against the Fae. Yet it was Feyre - not Nesta - who killed an innocent Fae with hate in her heart.

The IC hated Nesta for being cruel. Yet it was Feyre - not Nesta - who caused an innocent guard to be whipped. Feyre didn't check in on him afterwards because she cared or because checking on him was the right thing to do. She visited him to make Tamlin look bad. (I don't think Feyre understands that ACOWAR doesn't just make Tamlin look bad. It makes her look bad too.) It was Feyre - not Nesta - who displaced innocents out of their homes because they committed the crime of living in the Spring Court or living in the same apartment complex as Nesta.

The IC wanted to punish or execute Nesta for giving Bryce the Mask because they thought her actions would benefit the enemy and risk Prythian. Yet it was Feyre - not Nesta - who benefitted the enemy by destroying an entire court on the cusp of war. It was Feyre who gave the enemy a stronghold in Prythian. It was Feyre who risked Prythian by releasing a death-god and Fear itself. It was Feyre who lost control of Bryaxis.

Feyre is the monster the IC fear Nesta to be.