It sounds more like a case of Demonic Possession to me. Her emotional state prior to committing the deed was not mentioned. However once the deed was done, and the Demon, his mission complete, left her, she appeared to be remorseful and sorry. But it was too late. Two lives have been changed for the worse and one has been destroyed. The Demon did his job well.
No...she was regretful.
Regret is when you're concerned about the consequences of your actions to yourself.
Remorse is when you're concerned about the consequences of your actions to others.