Faceslapping Season
Having drawn out his gun, things escalated from zero to a hundred in a split second. He wasn't an unreasonable person but they just had to stir up shit for no apparent reason. They ruined his mother but he knew one can't force love so he accepted things as they were and helped his mother recover.
They took his inheritance after his grandfather's death but he didn't do anything. Yes, he knew about it but he had a very good reason not to fight back. It was because he wasn't willing to work like a donkey and keep this vile family's pockets fat and full. He wasn't stupid. He would rather watch them destroy themselves than work for them.
And now their brat had taken it upon himself to screw with someone very important to him so how could he let them go unscathed? This would teach this little asshole to stay as far away as possible from his business.