The son was willing to commit murder to get firearms. He shot his own mom in the face while sleeping, after all. If she had locked them, he seems perfectly willing to have tortured the location of the key out of her. Or are we to suppose she would have let her throat be slit with a handy kitchen knife without talking?
And, if the loon hadn't gotten a gun from the house after committing murder, one then supposes he was unwilling to murder someone else for their firearms. Or, simply do like the drunken loser who killed 87 people in the Happy Land Fire and get a gallon of gasoline.