One of the best villains in any fiction, he was just a guy you loved to hate.
In the original timeline reality, Biff Tannen became George McFly’s boss and still his bully.
Marty should have not been surprised, to see his young father being bullied by Biff all those years ago.
We later learned he’s actually pretty okay with murder. He also should go to jail for other things.
We also knew how much he hated manure.
After Marty gets back from the past, Biff becomes a pathetic car washer guy.
Old Biff Tannen in 2015 realizes that Marty and Doc Brown are time-travelers, so he uses the time machine himself to give himself a better future.
And he becomes a rich tycoon, and the only person happy in his future is him. With some luck Marty manages to fix this timeline.