I believe the correct answer is hiding an important secret.
Although sinister does mean evil, or intending harm, I believe that Poe didn't use that word his with that meaning. This is the excerpt of when he met the man on the staircase:
"His countenance, I thought, wore a mingled expression of low cunning and perplexity. He accosted me with trepidation and passed on."
There is no evil feelings here - the narrator can only see that his host is afraid because of something, and he doesn't yet know why.
