Oh. I'm sorry, I guess I'm not sure what you are asking... As I understand it, Williams might have faced execution if convicted on the original charge, but he plea-bargained it down to a lesser charge. If he would have still faced execution anyway, there'd be no plea at maybe and maybe he'd have walked free.
Are you asking if the world be be a better place if we didn't allow people to plea down on capitol crimes? Or are you asking if the world would be better if, in this case, he somehow got convicted of the higher charge and then executed?
I assume you are asking about the second point. Personally, I'm very fond my carbine so I might be biased, but yeah, I think people should be executed for killing law enforcement officers if they are found guilty fair and square.