What does the phrase "seduced by insanity" mean?
Seduction denotes the act of enticing someone from appropriate behavior, so to be seduced by insanity can mean that the attraction of the aberrant behavior and/or thinking is so strong that a person embraces it in spite of knowing that he/she is going to do something that is wrong; the pull of insanity is so strong that the person succumbs in spite of his/her judgment.
Much like the seduction of evil, insanity can take hold of a person after he/she has ventured into "the territory" and there is no turning back. Many of Poe's stories exemplify this almost magnetic pull of insanity upon the narrator. For instance, the narrator of "Tell-Tale Heart" has killed the old man with impunity, yet in his madness he reveals his crime. Before doing so he says, "...what could I do? I foamed--I raved--I swore!...I felt that I must scram or die!"