Briefly had that idea go through my head too. What was Devlin's history before his adoption. Given his age, the effects of what a child goes through during an adoption were not understood that well. I was pondering that maybe he was just messed up enough to think that he needed to "adopt" a child and this is how he went about it.
Do we know at what age Devlin was adopted? Not everyone who learns that they are adopted adjusts easily. Some of them struggle with feelings of abandonment and inferiority. I have no idea if any of that would apply to Devlin's situation, but I suppose it could. It definitely doesn't excuse what he did.