This is a very dangerous argument, at least to Catholics. Right now, the Catholic Church condemns the practice of capital punishment because we have other means to permanently remove deserving offenders from the public. If you introduce the idea that there should be no life imprisonment, you re-introduce the practical need for capital punishment to the Catholics
Pay no attention to the Old and New Testaments. As long as other means exist, indeed, just re-write the Bible. Stuffy ol´ stuff....
Despite what some people say, I think you are overstating the case. Pope John Paul II was personally against the death penalty, and may have a case, but he didn’t make it an obligation on Catholics to be against the death penalty, nor could he.