First of all, you are just hearing his side so who is really shooting off his mouth?
He claims he killed the guy as he witnessed the assault. Who knows what he really saw or whether or not he was complicit in some way. For all anyone knows the other guy could have seen the father molesting his own daughter.
I love how some here say if you are within your rights, all is cool when you agree but when the lawyer who didn't rat out his client because of privilege and allowed another to remain in prison (also within his rights and professional obligations, he was bad and immoral).
I somehow doubt the law was intended that once you had prevented the sexual assault from occurring, you could then continue to beat the guy until he dies. I suspect the lawmaker meant that you can use deadly force to prevent it but not once it is prevented that you can go on to kill the alleged perpetrator with impunity.
However, things are a tad different down your way.