I think you mean Montresor instead of Fortunato. I'm not sure what you mean by "is it necessary", but from Montresor's point of view, it is very necessary. The story is told in first person point of view, and Montresor makes his case to the reader why he feels it's necessary. Also consider his family motto and coat of arms--anyone who does harm to the Montresors will be punished. He feels it's also his family duty to punish Fortunato. The offense incurred by Fortunato doesn't matter because he's done something that makes Montresor think that Fortunato has wronged him. The fact that Montresor feels offended is all that's necessary for him to believe Fortunato must be punished. Whether he's insane or not is left up to the reader, but he certainly spends time trying to convince us he isn't mad.