Does Facebook need a "Dislike" button?

Nope. If you don't like something on FB, you have the choice to simply go on without clicking the 'Like' button. It's not necessary for you to like or dislike everything, everywhere, including FB.

It's like if you don't like cotton candy and saw someone selling them in a mall, you wouldn't just go up and voice your dislike. You'd simply ignore them and head to where the ice creams were.

Sometimes a "dislike" button would be useful, such as when a friend posts about something bad that has happened to them. When that occurs, it seems tactless to click "like", but also seems a bit heartless to just ignore what they've said. A "dislike" button would solve that problem.

No, I think that would just invite trouble. If you don't like something, then don't "like" it.