To keep it short. Steven's always lived in Rose's shadow, selflessly putting others before himself to adequately fill the hole she left. It's a subversion of the chosen one trope, where Steven has to spend the entirety of the main series desperately attempting to validate his existence. But Rose was never as great as people made her out to be, and the finale ends with Steven needing to prove he's his own person, not simply Rose in a new form, despite this proof being wholly dependent on whether or not Rose chose to sacrifice herself for Steven's existence.

Finishing the main series, I was skeptical about whether a follow-up was essential or even a good idea. However, Future takes a direction I would've never predicted, and it resonated with me and was a welcome follow-up to the main series. Future is mainly about Steven experiencing an identity crisis. Steven wants to be his own person, doesn't want to live in Rose's shadow anymore, has strong conflicted feelings towards The Diamonds and Rose, and is frustrated that he was put in this situation where he needed to justify his existence and was thrust quickly into the adult world.

Steven also finds meaning in his altruism; as a "chosen one," he finds meaning in allowing others to rely on him. He desperately wants to continue allowing said altruism to define him, often attempting to help people that don't necessarily need it, and gets frustrated whenever his attempts to help don't work. This climaxes with Steven, despite attempts to ignore it, being confronted on the whole situation resulting in him becoming a monster. (Some have said that to them, this represents the show saying that people in poor mental health are monsters, but to me, it's simply representative of the fact that Steven perceives himself as a monster, and unfairly so.) This is resolved with everyone attempting to help Steven, telling him he can rely on others and doesn't need to define himself by allowing others to depend on him. After this, Steven calms down and decides he will try to figure out his own identity; nothing is definitively resolved; simply, despite attempts at compartmentalization, the problem is recognized.

Also, some view this as the show portraying someone with mental illness as violent; I personally view it as someone slowly losing their support systems and, in desperation, accepting poor guidance.