Well...I think the article begs the question...of course the idea of capital punishment should be only in those cases where the defense of society is at stake. The question is one of scope! Many murders take place in the prisons by those who have murdered but are there for life because of the rejection of Capital punishment in their cases. Had they been executed, those who would have been otherwise murdered would have remained alive!
The problems arise in applying capital punishment in a way that those innocent but wrongly convicted might be spared. Hence imperfect though it is, we may well have to live with the cycles of appeals and re-appeals to make sure that each prisoner gets his or her fair shot in the system!
If you're looking at Old Testament law (thou shalt not kill) then there is clearly a difference between executions and murder. There were multiple words for "kill," each with a different meaning. The commandments use the verb for "murder an innocent," not the verb for killing in war, execution, self defense, or killing an animal.