I'm going to tell my flatmate she's rude because she wants to move out to her new apartment before our lease expires
Why do you think that's rude?
Because she didn't ask me for help to move out, she just decided by herself to do this thing.
When someone does something that hurts you, it's natural to be angry, but there's a fine line between healthy and unhealthy anger. Talk to them about it and see if they will honor their commitment on their lease.
I still think I'm right.
You are right that she should not have broken the lease with out talking to you about it. Maybe she will at least honor the amount she owes for rent.