We can tell murder is wrong because we have objective criteria by which we can determine for certain whether murder is wrong, and we can do that regardless of what mood the gods are in nowadays. There has to be an objectively verifiable reason why something is moral or immoral, or else no one could know whether it is, including God, since the Bible has Him changing His mind all the time. Without that criteria, you couldn't know what is moral or immoral either; you just have to trust whatever the people said who pretended to speak for God.

Youtube - The Damn Commandments (January 7, 2015)