It is of course no surprise that the anonymous narrator himself becomes affected by Roderick's rather bizarre and depressive behaviour after the death of his sister, Madeline. Note how he comments upon his own change as a result of the amount of time he is spending with Roderick without any other human company:
It was no wonder that his condition terrified--that it infected me. I felt creeping upon me, by slow yet certain degrees, the wild influences of his own fantastic yet impressive superstitions.
This is of course key to establish the Gothic nature of the text. One of the key components of Gothic literature is the way in which it uses unreliable narrators to create a sense of disturbing uncertainly about the presentation of what is supposedly "real." The way in which the narrator himself admits that he becomes impacted by Roderick's gloom and doom makes us wonder about the accuracy of his account and the rest of the fantastical story that he narrates. Does it really happen, or is it just the result of his frenzied and darkened imagination as a result of having spent so long with Roderick? Such uncertainty is key to the unsettling feel that all Gothic texts display.