On the other hand, M5 is Richard Daystrom in silicon. And he/it is crazy as a cuckoo.
Kirk has overheard Daystrom speaking of "civil and moral laws" a two distinct things, even specifying that such things are thousands of years old (which civil laws in a democracy aren't, because constantly tinkering with them is the only way to have democracy in practice). It's a good bet of Kirk to assume that Daystrom believes in "laws of God and Man" that were abandoned by the civilized world back when they abandoned both God and Man. Kirk then just goes and picks the one that fits the tactical situation.
"Murder is punishable by death" then is a convenient untruth that M5 strongly believes in; Kirk might just as well have said "Murder sends you to Hell" if he felt he would benefit from M5 retreating to its idea of Hell, or "Murderers have to do fifty push-ups" if he felt M5 believed that and would blow up out of shame for being unable to do any push-ups at all.