Justifiable homicide. The stepfather needed killing.
I generally agree with your sentence, but I am a bit squeemish about the execution style (shotgun to the BACK of the head) and the following dismemberment.
I believe that cases like this can lead to other miscarriages of justice when a woman shoots her mate while he's sleeping on the couch. If they're in an abusive situation, they can walk out the door. There's a lot of places out there that will take them in. This girl was 19 at the time of this incident, and should have been able to leave the home and live on her own.
This should be a slam dunk. The girl’s act was simply self defense. That her father had pictures of the long standing and frequent abuse should have led to the prosecutor not indicting her.
That the case has been pursued this far is a travesty. If it were me I’d find a way to indict the prosecutor for aiding and abetting a sexual predator.
By all means. Kill the miscreant. And if anyone objects, just pay the wergeld.