I think the man had a right to defend his daughter, but I think it's appropriate that he did have to go through some legal proceedings over the assailant's death. I think no killing is ever truly excused, even when it is deemed justified. I'm fine with him not being indicted, but I'm also glad he was not simply let go off-hand, without even a proper review.
Those who say the assailant was "broken", sure, we know, at present, very little about effective treatment of pedophilia, but jumping straight to death being the only way to deal with such things is quite a leap. There are surely many with those feelings who manage not to act on them, and surely a few who have acted on them who have been, if not mentally, at least behaviorally remediated. And not every abused child is scarred. I heard a great piece on the radio about how telling children they MUST be and certainly are horribly scarred if someone takes advantage of them often does them as much harm as the abuse itself, because it denies them the ability to attempt to control the direction of their life after the abuse.
I don't have an answer for dealing with child sexual abusers. The reason the topic is so contentious is very few people really do have good answers. Those who unilaterally call for death, or locking them up for life, often go too far, as I think those punishments are overused in our society and as such have lost much of their meaning and deterrent effect, and both are also a pretty expensive drain on public funds. But I also don't know what lesser ameliorative options there are. Castration is generally ineffective because the problem is often more about power and obsession than sex. Treatment, thus far, is inadequate to the task, in part because we have so few tools and such poor understanding of the underlying disorders. This isn't helped by the way we socially stigmatize those who have such feelings, even if they are perfectly capable of keeping them in check and not acting upon them. Should we punish folks for thoughts without associated actions? And yet we do, and doing so makes it so much more difficult to find ways to treat or manage the disorder for folks who can't help but act in those impulses.
In a society like America with a justice system that values fairness, protection of rights, and the prevention of cruel and unusual punishment, but which also lacks any real mandate to rehabilitate or remediate, issues like this are incredibly complex.
I understand why so many feel so justified by the father's actions and the man's death, but I also think that the ease with which we accept this as a normal and acceptable response is also troubling. For the father, he gets hit twice. His daughter was sexually abused, but he also killed a man. She's much more likely to have a healthy recovery than he is. Patting him on the back and telling him the attacker deserved to die because he was "broken" isn't going to help the father feel better.