It is interesting how quickly I went to legal matters and you went to personal ones. I think that speaks volumes to our respective cultural conditioning.
There is, of course, a personal element to the Jewish understanding of the Third Commandment. Like many people today, it was customary for the ancient Hebrews to bolster an argument with reference to God: “If this isn’t the truth, may the Lord strike me down where I stand.” Such statements can easily be abused, and this may be why Jesus says, “Let what you say be simply ‘Yes’ or ‘No;’ anything more than this comes from evil,” (Matthew 5:37). Which is a good segue to your point about God not endorsing evil.
You are right, of course, to point to those people of faith who used their faith to legitimize slavery, but can we really say this was a misuse of that faith? There are dozens and dozens of references to slaves in the Bible, and while the Torah clearly stands against the abuse of slaves it does not condemn slavery, but endorses it as a part of everyday life.
Nor is slavery the only evil God endorses. Genocide is clearly God’s intent with regard to the Amalekites: “Now go and attack Amalek, and utterly destroy all that they have; do not spare them, but kill both man and woman, child and infant, ox and sheep, camel and donkey,” (I Samuel 15:3). And then there is the matter of the Flood, the murder of the first-born Egyptian, and the slaughter of tens of thousands of Israelites who challenged Moses’ dictatorship in favor of proto-democracy, to name but a few of the more troubling aspects of God’s morality. The Bible isn’t exactly a guide to liberal democratic values.
We can, as the history of Judaism and Christianity shows, make the Bible say anything we want it to say. This is why I cannot rely on the Bible for moral guidance. The text is too inclusive of different authors, times, worldviews, and cultures to give us a clear read on morality. Everything comes down to interpretation, which, regardless of the origin of the text, makes it an all too human tool.
I have to pick and choose which biblical teachings to follow and which to reject. I have to decide for myself what is good and what is evil both in the Bible and in life. And while I find great wisdom in Micah 6:8: “Do justly, love mercy, and walk humbly with your God,” I know that my take on justice, mercy, and humility is my own and not that of the Bible itself.
So I agree with the Third Commandment: don’t use God to strengthen your own opinion; and I agree with Jesus: don’t use God at all.