At the close of Chapter X, what analogy does Nelly use to describe how she felt about Heathcliff’s presence at Wuthering Heights?

From the text:

His visits were a continual nightmare to me; and, I suspected, to my master also. His abode at the Heights was an oppression past explaining. I felt that God had forsaken the stray sheep there to its own wicked wanderings, and an evil beast prowled between it and the fold, waiting his time to spring and destroy.