What do you mean when you say "God is Good"?

This concept is so obvious that nobody seems to get it. It doesn't matter how powerful a being is, or how ancient, or what dramatic deeds he may have performed, he doesn't get to declare himself "Good" (or any of the other adjectives attributed to him) by fiat alone. If everything this being does is, by definition, "Good", then the word no longer has any meaning. If God is automatically "good" without reference to any objective external moral standard, then ANYTHING he does, no matter how obviously heinous and evil by human standards, is nonetheless still good. It becomes a mere tautology to say that God is "good", and utterly meaningless to even use the word at all. Using this standard, I can justify any evil behavior I might commit, simply by saying "How do we know that God would not do the same?". Based upon this standard, no human being has the right to declare ANYTHING good or evil, since in their limited human understanding they have no way of knowing whether or not God would do the same. They can no longer say "Well, God wouldn't do that, because it's evil." I can simply counter, "Well, if God did it, it would no longer be evil. So therefore it is good." And also, how do you KNOW that God would not do it? If you read your Bible, you will find where God has ordered or directly committed some of the most heinous, atrocious acts imaginable (by human standards that is). Genocide, slavery, smashing babies against rocks, gutting pregnant women, killing children for the misdeeds of their parents, wiping out entire towns wholesale, including women, children, babies and even livestock, asking people to sacrifice their own children, drowning an entire world, burning people alive for eternity, and on and on. And yet believers always seem to give him a pass for these things. Nobody ever has the courage to take him to task, to demand an accounting of him for the obvious evils he commits, and allows to be committed, every second of every day on this planet, and for every moment of eternity.