I think it's most often used in media, for example movies or books, as a trope where the character who is a comedy sidekick or the butt of every joke gets sick of it and shows the people who were laughing at him another side of his character. Maybe he becomes a villain, therefore showing the people who laughed at him that he is much more than that and is victorious in the end and now he's the one laughing and "having the last laugh" and then the "who's laughing now?!" moment comes in perfectly. Hope this helps!

That's right !!! This expression is generally used by spiteful people, or people who are going through a bad situation caused by others.

Yes! It is like revenge