So, What Part of Capital Punishment is Ineffective?

A man murders someone, serves 14 years, gets out... and kills six more people. 6 more.

See, now here's where I'm thinking that the death penalty, if applied in the first case, would have been 100% effective. And six more people would be alive.

Who are the next 6 who will be murdered because a known murder was released from prison? At what will your argument be if you think the death penalty shouldn't have applied to that murderer?