A recent item on the news told of a case where a man broke into another man's house and threatened his family.

... It went badly for the invader and he fled.  But the man who's family it was chased him and beat him severely causing permanent damage.

The invader was convicted naturally, but the other guy was threatened with almost as harsh a sentence for beating the guy, as it was deemed unreasonable and excessive force.

I think everyone has sympathy for the man and his family in this situation and all too frequently hear such stories. (and most would do precisely the same regardless of consequence if placed in that situation!)

But of course the law must be blind and still punish any action which in itself is contrary to law (vigilante justice etc.)

But there is no escaping the fact that the guy who's house was invaded would never have had cause to commit his "crime" had the invader not commited his offence in the first place!

So the invader is originator, and therefore AUTHOR of both crimes.

This should be reflected in the law so that whatever sentence the family man recieves for his "crime" is halved, and the invader takes the other half of the sentence as recognition and penalty for causing it to occur.

So this means that the potential criminal now has to assess the potential dangers of commiting it in a whole new way, as his stretch in prison could be extended depending on how severely the victim responds! 
It could become very costly for a potential criminal under this method. and give the potential victim more protection in law, without compromising due process.