Q: If I wanted to make a free fancomic based off a certain Marvel superhero, would that be a violation of copyright ?

A: Fan fiction is generally considered derivative work, which only the original author may lawfully produce. So, you can be sued for producing fan fiction. Actively making money from writing fan fiction merely makes you more of a target for being sued, however, it's not a necessary component to a successful copyright infringement lawsuit.

Your legal exposure is going to depend almost entirely on the author's (or the intellectual property owner of the author's work) stance on fan fiction. Writing in a universe created by Joss Whedon or J.K. Rowling? These authors actively encourage fan fiction to be written about their work. Writing about Gone with the Wind? The estate of Margaret Mitchell is notoriously litigious.

The minimum statutory damages for a copyright infringement suit (in almost all cases) is $750 and damages can go up to $150,000.

The doctrine of fair use is invoked as permitting almost anything by anyone who has done only a little bit of reading into copyright law. Fan fiction can qualify for fair use, but, based on what you've described this will only really help you if your work critiques or critically analyzes the original work. A court is the only entity capable of making a determination of whether fan fiction qualifies for fair use - and only after a lawsuit has been filed.

The minimum statutory damages for a copyright infringement suit (in almost all cases) is $750 and damages can go up to $150,000.

Additionally, many Marvel characters are covered by trademark law in the realm of comic books, comic stories, and artwork. So you could also be liable for trademark infringement and other associated legal claims.

If you're interested in reducing or possibly eliminating your risk of legal liability, consult with an attorney who focuses primarily on copyright law.