Beth said, "I get it now," hence my question, what did she 'get'.
She "gets" that she was being abused, though Dawn tried to make her feel like a friend.
I think that is all that Beth's reaction was supposed to be about: an abused person lashing back at the abuser. She couldn't and didn't predict getting shot or the group's reaction. But she was a girl who was maturing and growing and wasn't going to walk away from being treated like that. Having Noah taken back, after she helped him escape, was her last straw.
I'm surprised nobody else here was concerned/commenting about Dawn's hand being on the trigger of the gun. From the look on her face after shooting Beth, she didn't shoot her intentionally. They used the idea that she had her hand on the trigger to make it a somewhat accidental discharge.
What I wanted to hear during the verbal exchange (ie regarding Dawn asking for Noah back) was someone voicing the obvious that the cops weren't seeing: they were effectively keeping slaves. Rick's group took hostages only out of temporary necessity. The hospital group was using slaves for their own survival.