Yes, sometimes a pattern of abuse is the cause of a crime. But dismissing all serious youth offending as caused by the offenders own childhood regardless of whether or not there is much indication it was - well that does, and did in Venables case - mean that other causes of the offending will be overlooked and not treated/dealt with, which means they reoffend.
That is a very tricky area... on the one hand, it stands to reason that any childhood is a factor in how someone turns out... even if it's not a particularly troubled environment. A childhood still shapes a child.
In the case of Venables, the fact that he's continued to do bad things doesn't necessarily mean his causes were something else, it may just be that his rehabilitation didn't work. Clearly it's not worked well, that's for sure. But seemingly Thompson's done 'ok' (we might be unaware of smaller issues... I'd be surprised if there were none at all), likewise Bell's managed to make a life of sorts - so I think that gives hope that sometimes a child can come out the other side.
With Venables there has to come a point where he's used up all his goodwill tokens. He's probably well past that point already, but that doesn't mean we can't look back and see causes. That said, we can never be 100% sure of any specific cause, just take a decent guess at certain factors that might have influenced a child who was already predisposed to certain behaviours.
We only need to look at two siblings to realise how different their personalities can be despite the same environment - but that doesn't mean their environment should affect them in the same way. It's all so complex to decipher.
Still, I would always like to hope we try to rehabilitate rather than just kill. But how many attempts can we make? and if we don't rehabilitate is that because we don't know how, or because they cannot be rehabilitated?