No.6 Discussion Board

Funnily enough, @Weisel's icon is a constant reminder of that ;)
It wasn't just the Rikiga scene though. Shion already showed violent tendencies when he was twelve ("Destroy it all" and such). It's like ... well, at one point in Volume 7, Nezumi wonders when Shion lost his fangs and claws. I think they're just buried very deep and all the stress in West Block and the hostile environment brings that forth in Shion. He protects the people he loves no matter what and everytime he lost control, it was just that: Protecting loved ones (namely Nezumi. We never got to see Shion protecting someone else but I'm sure that wouldn't end up significantly less violent).
It's there. Shion might not have these instincts for himself but he sure as hell has them for others.

Nezumi's behaviour and words just gave Shion a name for it: Punishment. Nezumi was appalled that Shion would go so far but maybe he was just as appalled that he would use his own words to justify nearly killing a person. And maybe he finally started to see that he has had an influence on Shion after all and to wonder what else he picked up in the disguise of his pleasant personality.