You may have witnessed Jack Ruby shoot LHO, but you may not have had all the facts. He could’ve been, for example, a highly paranoid schizophrenic suffering from an intense delusional belief. In which case you may have seen him undertake an action but have no culpability. That person may have been found ‘not guilty by reason of insanity’ here in the U.K, it’s a pretty extreme example I grant you, but actus reus and mens rea are separate things and both are important.

In the case of Jack Ruby though, what you saw was what you got - and the process is important.