I just don't get how some people, especially people called martial artists think. I just don't get it. Mr. Canna's scenario is about as straight forward as it gets and yet we pontificate, beat around the subject and mentally masturbate it to death.
1) We live in a society. We have an implied social contract. If one breaks that contract we cannot trust them and the deal is off. Five guys with bats on a lonely beach wanting to rape my daugther doesn't really call for heavy thinking. I have no belief at this point that these people are worth/able to negogiate with. Sanford Strong's 'second crime scene' is directly to this point.
2) Intent and Ability. Massad Ayoob 'In the Gravest Extreme'. If the attacker shows intent to perform grave bodily harm and has the ability to perform that harm then you are in a defensible legal arena. (a situation I would happily be in given the scenario. You can't bring a dead man to court).
3)See above. Disparity of force. Definitely applies here.
4)Stupidity factor. I am not implying any stupidity here, it is just a term. The point is that I do not put my children in any situation without taking reasonable precautions. I don't know the details, however I have taken my family on walks on the beach at night, camping, and long drives through deserted roads. In those scenarios I have taken the responsibility of their welfare seriously and prepared in a reasonable fashion. Again, I am not implying anything. What I am saying is that in the given scenario, that man and his daughter have limited choices, none of them particularly good.