She is sad for betraying Ravenna, her homeland. Alba wanted to return home after running away two years ago and show her family what she had done through that time, explore and living adventures with new friendships.
At least Alba stayed with her new friends, right?
If we’re talking in the context of regicide, then she did not betray her country.
In fact, this is probably the most patriotic act that one can commit to a country, by killing a corrupt monarch, they are giving the nation a chance to rise and topple the monarchy, and form a Republic.
Still, you can really see the sorrow and guilt in the face. Well done.