Well maybe we are judging the guy too harshly.
Maybe the fence owed him money.
Maybe the bloke had come home from a hard day at the muggings, to find the fence in bed with his girl.
It could even have been that he was just angry at his father, who abandoned him and his mother when he was a kid and the Dad returned to be a part of his kid's life, cleaned up, off the drink and working again after getting out of jail, he erroneously believes this will be enough to esuage the years of ingrained disappointment and anger felt by his son finally gains the confidence in his new job as a concrete fence to step forth to make up for the lost years....oops.
It might be that the guy viewed his own leg as 'the enemy'.