Evil Steven, trapped in an endless time loop he never understood, he saw all he loved destroyed again and again. And that was only the beginning, with a multitude of different people endlessly tormenting him, making him suffer as if for some horrible crime he had only the faintest glimmer of. Until the multiverse troubles started.
When a dimensional rift freed him of his curse, Steven found a Beach City that was eerily similar to his own. At first things went well, only there was another Steven. At first the Gems of this universe accepted him, but Loop!Steven was forever changed by his experience. He was a lot more brash, and prone to anger.
He also went out of his way bullying Connie. When Steven tried to stop him, he got angry and killed Steven in a fit of anger. Loop!Steven was then imprisoned by the Crystal Gems and Greg. Feeling betrayed by the ones he loved, Steven went utterly bonkers and broke out of gem!Prison, destroying Beach City and killing the gems.
Now the Mad Diamond wanders the multiverse, devouring worlds. His only weakness being his irrational hate of Kevin and the many other apparitions who tormented him... Especially if they're on good terms with 'their' Steven.