"Why waste court time when you've got the bloody knife in your hand"?
Seriously?
This pretty much invalidates your entire opinion. I can concoct hundreds, if not thousands, of scenarios in which someone will look guilty in your eyes and yet be completely innocent. In fact, the writers of the various CSI and Law & Order franchises do (did, I guess) this every day.
You can't execute innocent people. We do it all the time in this country. That has to stop, and if it means more sadistic murderers end up in prison for life, I am completely fine with that.