It is already written into the law that when someone kills someone else, their ultimate crime is determined by their mindset. In many cases the person is not charged with murder, such as the guy who shot his daughter's rapist. Manslaughter is in place for cases like this, and so is negligent homicide.
Raping a girl/woman is something that leaves lasting effects on that person. When you consider that, the idea of a father shooting and killing the person responsible for his daughter's permanent mental and possibly physical torment isn't as heinous. It doesn't mean that people should be allowed to just take justice into their own hands, but I think there needs to be an emotional consideration when sentencing the father.
Vigilante justice is a touchy subject and is frowned upon but it should not be punished the same way as a senseless killing. When a man kills his daughter's rapist that is merely him bringing justice that the courts would probably not give. I know we have laws and systems in place for a reason but sometimes the system fails and we need to take matters into our own hands.
Yes, there should be common sense applied to these types of rulings. However, there needs to be a balance. Criminals deserve protection too, and we don't want to turn into a society of vigilante justice. I can see both sides of this issue, but the type of crime needs to be considered.