Spanking someone else's child...please read whole story before you judge.

About a year and a half ago I spanked my nephew.  It all started the day my son was born.  Ever since he came out of the womb, my nephew(Bobby who was 2 at the time) did everything in his power to hurt my son.  Playing with baby toys, Bobby would smash my sons fingers on the toy or just do whatever he could to make him cry and physically hurt my child.  This went on for two years.  I kept my eye on the two boys the best I could but you can't catch everything.  Bobby got to where he was so conniving that he knew to look around the room before he attacked my son.  My son got his first bloody nose at age two because of Bobby.  Finally one day he was over at MY house.  The boys were playing in my sons bedroom and I went to check on them.  And I fine my son head down in the carpet crying because Bobby is on top of him punching his ribs and his back.  That was when I snapped.  I grabbed him off my son, spanked him and told him to never lay a hand in my son again.  Now, his mother thinks I'm the devil.  Please give honest input.  This has been eating at me for a while.