Would've been funny if his dad was seeing him as untested and unready so he brings the criminal out and before he's even done talking about sentencing, Ars walks up and jams his knife into the guys neck. Turns to dad, with a big grin and blood on him and is like "I took out the trash father, what's next?" lol Dad would be like shit, now I'm concerned about something else
On a more serious note, dad picked a bad choice anyway. If I wanted to test the mettle of my son in this kind of thing, I wouldn't pick a stereotypical scum criminal. The dude going on about whatever it was he was saying (can't remember now) would, if anything, make it easier for the kid to do it. I'd try to find the most benign, innocent looking criminal that would weep and beg for mercy.
That would be a real test of his mental fortitude.