Because many of the creeps who break into homes own them as well:

Though this happened in Chicago where, to my knowledge (anybody else know differently?), guns are still outlawed it seems the police are being decent enough not to pursue any charges against him. Of course we all know what would have happened to this elderly man and his family if had lived the liberal dream and given up his evil guns long ago. They'd be dead... pure and simple.

Here's what the elderly man's son said about exactly how everything happened:

The son said the intruder, armed with a pistol, came in through a rear window and ran up a rear staircase, banging on his locked door before running downstairs and being shot in a confrontation with the older man. The intruder shot first before the veteran fired back and killed him, the son said and police confirmed. “Evidently, he missed,” the son said of the intruder. "My father had no choice. It was him or the other guy.

I'll leave you with this:

A couple of months ago, the man — a Korean war veteran with three children and six grandkids — had been robbed at gunpoint at his home by three intruders, his son said. The robbers took $150, he said — and his father bought a gun and vowed never to be a victim again. “If homeowners can’t have guns to defend themselves and their families, there’s going to be more home invasions,” the son said. “My father’s glad he had a weapon. He did what he had to do.”