I thought that was very sweet. Steven is not a killer. Even to a giant life form ready to tear Earth apart. Steven saved the Cluster just like he saves every Gem like his mother before him: by bubbling it and waiting for the day he can restore them and bring them back to their former glory.

The Cluster didn't even want to destroy Earth (did you hear all those little "no's" when that was mentioned) and Steven realized that. Poor thing was just lonely and in pain. Hopefully now that it's bubbled it can come to terms with itself and work something out. Maybe even some of the shards can find themselves.