The point of no return has been hit. There was a chance Tenko could’ve gone down the right path. A chance he could recover from his trauma in a healthy way and go on to be a productive member of society.
But, not on All For One’s watch.
People can be crappy. Yep, as mentioned by Shigaraki before, no one tried to help him in his time of need. Like seriously, there’s an f-ed up looking kid walking around by himself and no one called the police? Damn. Well, that was society’s last chance to prevent Tomura Shigarki’s birth, and they failed.
All For One makes it very clear how he wants Tenko to think. That’s what’s so insidious about him. It’s complete manipulation on his part in turning Tenko into Shigaraki, but it’s in a similar way to a parent teaching their child. But, instead of teaching your kid to be a good person, you tell him “Nah, screw the rules. Do whatever you want.” And, what happens when you tell a child who holds onto anger and sadness and murderous intent like it was a new toy to do what they want? Dead street punks. Specifically, the kind who earlier beats up Tenko. Why do I have the feeling All For One was watching Tenko kill his family the same way he watched him kill those punks? What a jerk!