In 1991, a reformed William Baker, a.k.a. Flint Marko, a.k.a the villainous Sandman, joined the Avengers to battle evil. His tenure was brief and tragically he turned back to his criminal ways, proving that some comic book writers don't believe a leopard can change its spots. Still, while his membership lasted, Sandman's inclusion provided hope that villains could be allowed to try and atone for all the harm they'd done. Which is the sort of moral comics and movies should have. Why he won't be in the Avengers movies: Sandman has already starred in a blockbuster movie, Spider-Man 3, played by Thomas Haden Church, meaning that movie goers are already familiar with his origin and history. Whilst that helps his case for a return appearance in a story where he turns to good and seeks redemption, there are a couple of factors acting against him. Firstly, even though the rights to Spider-Man are being worked out, it's not clear how much of the "Spider-verse" can be shown in the MCU and, again, the whole "bad guy turning good" was done in Age of Ultron. The same issues could also prevent Venom from ever joining the Avengers on screen. Besides, in the case of Sandman, the atonement didn't stick and that's not the kind of message a director will want to put in a Marvel movie.
Mike has lived in the UK, Japan and the USA. Currently, he is based in Iowa with his wife and 2 young children. After working for many years as a writer and editor for a large corporation, he is now a freelancer. He has been fortunate enough to contribute to many books on Doctor Who over the last 20 years and is now concentrating on original sci-fi & fantasy short stories, with recent sales including Flame Tree, Uffda, and The Martian Wave. Also, look for his contribution on Blake's 7 to "You and Who Else", a charity anthology to be released later this year.
You can find him on Tumblr at https://www.tumblr.com/blog/culttvmike