He didn't want an innocent boy to die for his actions but didn't feel strongly enough to confess himself. And he did find a copy of that super-rare knife.
He was the one who stressed how horrible the kid had it at home. Maybe the guy's treatment of the kid motivated him?
Supposedly super-rare knife. The store owner who sold the kid the knife swore on his life that it was the only one of it's kind... And another one appears in a pawn shop, according to Juror 8. This in of itself proves the knife was reproduced, and can't be nailed down to one or two men. This in turns leaves the only evidence brought forth to be 8's strong notion of the kid's treatment.