Eliezer Yudkowsky's dissolving of the consequentialist/deontologist divide can be applied here, I think: Nothing is more precious than a human life. But people die, so we have to make compromises to minimize the rate of unnecessary death; there's no use spending all your time and resources to build a completely death-proof car if you then die of untreated appendicitis you were too busy to have fixed.

In addition to that opportunity cost concern, there's knock-on effects, like you touched on, mustpax. Letting a killer live saves one human life now, but a policy of letting killers live may end far more human lives in the future. Personally, I don't think the data supports that, but if it did that's a choice we'd have to make.