The husband felt he and his wife was in danger. There was no to be sure that the would-be rapist would not return and try to attack again. It is also clear that he was in rage (who wouldn't if they saw someone trying to rape their wife). While I don't think that it makes him a hero or that this behavior should be always encouraged, his actions were reasonable and he doesn't deserve a conviction for it.
No, this case does not encourage vigilantism because the husband was acting on impulse. In many ways, the husband could be regarded as a hero. Cases should be viewed on an individual level and given the circumstances, the husband did the right thing. The criminal justice system made the right call by dropping charges.
The legal system acted correctly in dropping manslaughter charges against the man who beat the man who attacked his wife. Justice requires weighing the facts, and this man had a right and a responsibility to protect his wife. Vigilantism occurs when people take the law into their own hands, meting out rough justice when they have no right to do so. In this case, it was a matter of protecting his family and home and not a matter of appointing himself sheriff, judge and jury.
The dropping of manslaughter charges against a man who beat his wife's would be rapist does not in any way encourage vigilantism. What the man did was purely for self defense and to protect his wife. He prevented a heinous act from happening and it is unfortunate the suspect lost his life.
This particular case was a unique one. He did not beat the man after he had left, he did it in order to stop his woman from being raped. This was not about vigilate justice, this was about protecting the people that you care about. It falls more under self defense than vigalitism.