I believe it was purely for reasons of expansion. I don’t know exactly what our response should have been, but we need to at least acknowledge that Ukraine is defending their sovereignty and Putin/Russia are the aggressors. We should support them at least minimally.
Yes, I do believe they feel justified
Controlling Crimea gives them a much better port than anything else around there, and a big naval base they developed around it.
A lot of the reasoning behind the Warsaw Pact back in Cold War days was a buffer zone. If WWIII did happen, they wanted to keep it far from their territory. Same deal. Now they imagine NATO right up on their border and that buffer zone is looking like a really good idea. In this scenario, they also have no reason to care whether that part of Ukraine is a bombed out wasteland