A daughter who commits a murder and uses rape as an excuse is definitely going to hell, unless she repents and confesses her sin and it is absolved by competent authorities. Thus, the only thing a loving father can do and should do is disown her if she commits such an action.
By threatening to disown he saves his granddaughter/grandson from being murdered and his daughter an eternity of regret. By actually carrying out the disown should his daughter commit the wicked act of Prolicide, the Father may hope that his extreme act jolts the daughter into awareness of grave act she has committed and that her eternal soul is in imminent danger.
A Father who loves his daughter above all looks after her eternal disposition.