Huh… Looking back to how she acts as she's being corrupted, this isn't that far off.

Though, at the very least, Jasper really didn't take any of her frustrations out on innocent people. It does make me wonder if Yellow Diamond or someone else might show up and just glass an entire city to piss Steven and the others off.

If an entire city is wiped out, millions of lives lost, etc., that would actually be a huge step for the show (maybe too big for Cartoon Network's comfort). There has been very dark themes and implications in many episodes, but rarely something so directly grim. While it's easy to argue the suffering of the gem mutants (the Cluster, in particular) is even worse than an entire city getting disintegrated, not only for its duration over time, but also its numbers, it's only fantasy, and it takes maturity to properly envision that horror. Entire cities becoming piles of dust is something that has happened plenty of times. It's the reality that makes that the grimmer possibility for the show.