People don't understand the ending...
Initially, I was infuriated by the ending and saw it as implausible. (I guess I that's what you get when you watch too many episodes of "Law and Order.") Then I read some of the comments by the director and grasped what he was trying to do.
The reality is, the mother was deliberately condemning herself to a prison sentence, both in the jail and inside her head. She was sentencing herself, independent of what happened, by her silence and through her actions.
Think about it. Had she euthanized her son and let everyone know, they may have let her off, but it would have done nothing to absolve her of the guilt and pain she was feeling inside. So she condemned herself.
The whole movie is about her life AFTER prison, but it makes the point that even though she has been released, she is still imprisoned. The performance conveys this throughout.
I'm not very good at explaining this, but I have to give the director a lot of credit for coming up with something different and subtle.
What a great movie.