Venables is a **** who doesn't deserve to live. But along with the short-term pleasure of knowing he is dead, there are negatives with capital punishment and most have been mentioned already.
You can also add the complication in this case that he was a minor at the time. Presumably no one is suggesting we execute children but that's part of the reason why he's been released earlier than if he'd done it as an adult.
Dying in prison at an old age seems fair to me, especially if he wakes up each day wondering if this will be the day that he gets sugary boiling water poured on his head.