One thing I think we have to remember is that what we "know" about criminal cases tends to be very much influenced by our own political or social beliefs. For example, there are many people out there who "know" that OJ Simpson killed his ex-wife and her boyfriend. There are people, like you, who "know" that Casey Anthony killed her daughter. And there are people who "know" that Troy Davis killed that police officer.
What I'm saying is that I don't know what is true about Troy Davis or Casey Anthony or OJ Simpson. I think that when we look at cases like those we look at them through the lenses of our own experiences and beliefs about the world.
That said, this is one reason I am generally against the death penalty. It is too hard to know for sure who is truly guilty (as we see in these three cases). And it is too hard to know for sure who really deserves to die.