Personally I think that the government should be less barbaric than criminals. At the very least, it shouldn't be even MORE barbaric. Yet that is exactly what you are suggesting. In the case of rapists, you want the government to kill someone who hasn't even killed anyone themselves. The punishment is even harsher than the crime. In the case of murder, you're at least on more solid ground in demanding their execution...but it's still rooted in emotion. It doesn't benefit society to execute them as opposed to imprisoning them.