The thing is, the guy who held up the family at knife point HASN'T got away scott free, he took a serious beating. If you factor in the vigilante justice as part of the sentencing then it's easier to see why the judge wouldn't pass a further sentence.
However the guy who dished out the beating did it of his own free will, admittedly provoked, but still, beating someone up is against the law and should be punished.
That's how I think the law works (or has done in this instance). The fact that I'm also firmly in the camp of burglars/intruders leaving their human rights at the door when they break into my house is beside the point.