I don't think many people would argue with that sentiment, as its fairly logical.
But let me play devils advocate for a minute, with your Keeping 100 alive to ensure you save one, statement.
Venables will be free one day. It might be two years, it might be four, it might be six, we don't know yet, but he will be free one day, that we do know. Now, hypothetically, what if he went on to reoffend, only this time it was someone close to you. Doesn't neccessarily have to be family, but someone you know. And through his new hypothetical crime, (which for all intents & purposes I'm going to say is extreme peadophilia & murder) he'd now affected & changed 100 innocent lives, including your own. Aren't those 100 innocent lives worth more than one murdering, nonce scumbag?
Now I accept that we don't know exactly what Venables will or won't do in the future, but his psychiatric evaluations, coupled with his past/present crimes, paint a picture of someone who will forever be a danger to society, especially young children. It is NOT beyond reason to assume that this person WILL reoffend, and given half a chance, would be willing & capable of another heinous, violent & shocking crime.
So, a simple question... Does his life mean more than 100 innocent lives?