Ah but that was exactly my problem - Dexter saw a pattern of knife work that he thought showed some "potential" so that is where the original selection started - agreed. But how he approached the boy was very clumsy and impulsive for a methodical kind of guy. Then he gets confirmation that boy is going to be a serial killer (or at least kill again) and he lets him off with a "Don't do it again" type of warning? It still doesn't gel for me, regardless of how appalling the cause of that impulse (the rape) to kill was.

Dexter of all people would know how powerful that urge can be - only someone with a guide or mentor like Harry could change (or, rather, redirect) the urge so the chances of the boy killing again are very high - unless that will be a later plot point for the series? The boy kills again and Dexter wrestles with the consequences of a rare moment of mercy?