What's in a Name?

 
His cutthroat razor gleamed ‘you raped and beat my mother, Johnson.’ Then he leapt, slashing. I sidestepped, kicking his groin.  
The razor at his neck I asked, ‘how old are you?’
‘Twenty, bastard.’
‘I left Eileen Brady twenty-one years ago. My name's Johnstone.’  
That’s how I first met my son.