What's with the Legal System?

Ok, you've heard the story before: Guy lives life of crime; Guy hangs with bad guys; Guys do crime; Guy gets Capital Punishment; Wrong Guy dies. This issue seems to come up quite often and opponents to the death penalty bring it up as a crux to the position; that anyone for capital punishment should concern themselves first and foremost with executing the innocent. Why then does the death of the innocent seem to have more to do with vengeance than justice?
When Ruben Cantu was executed in August of 1993, there were more than the voices of silent witnesses waiting for his death. There were public figures interested in death and popularity more than truth and light. Shouldn't cases like this demand the most concrete facts, more than any other? Well, in an article found in Houston's "Chrono.com", there is nothing but pain and regret from those directly involved, while those who had the power to make a difference from the start of the case talked business as usual.
How did it come to this? How can a seventeen year old spend his final years on death row without the case being seen through fresh eyes and a fine toothed comb? Aren't we dealing with the life of a human being here? Shouldn't the first thought of the Prosecutor be to administer justice? And what of death, should it not be approached as the final option? It seems, however, that the legal system with all its layers of bureaucracy will always kill the innocent, as the confessed killer sits on "the Row" twenty years and the lawyers collect their millions. I guess there's no money in freedom.