It's not like Jason has a leg to stand on when it comes to talking about techniques for coping with stress - his typically involve blood and broken bones at minimum, if not an outright body count. Getting a little snippy with a stranger is fairly low on the scale of offenses.

And something about what Billy says - I'm dealing the only way I know how. It's kept me alive so far. - something about that string of words resonates with Jason. How many times has he thought the exact same thing, or close to it, justifying his actions to himself? Hadn't he said more or less those same words to Bruce in defense of some display of excessive force on the job or other? Jason can clearly see the kid's scared, anxious, maybe a little sheltered -
probably never been in a bad situation like this before in his entire life. Jason had to learn the brutal realities of the world much too young, but even he knows that isn't normal, or at least it shouldn't be. That resonating, discomfiting echo softens the ever-present impulse to fight, to fight back, and Jason - he decides for once to let it go.

"Have fun being good," he says, and turns to leave. He's done here.