If it's the latter, then your proposed solution represents the creation of another problem... You can't solve a problem by creating another problem. Period...
Additionally, it recklessly neglects the safety of innocent lives. A system of law CANNOT prevent crime, it can only acknowledge the occurrence of crimes EX-POST-FACTO... People who commit violent crimes aren't acting logically or reasonably during the commission of a violent crime, what you're suggesting simply won't prevent rape, murder, or any other "crime of passion".
What it will do however, is contribute to the undue financial burden shouldered by the remainder of society to the point where that society will eventually and invariably dissolve itself.
The notion that a criminal "may" suffer at some point later on in the future simply isn't as powerful as the notion of immediate suffering. Passionate emotion (lust, fear, anger, hatred, etc...) doesn't overpower self preservation in the mental hierarchy...
You are right. Hanging these bastards after a fast track trial is the right solution. Quick, effective, and wouldn't cost too much.