Thank you, kind sir. I think you are the first person here to accuse me of using logic.
The Bible is our only guide to the character of God - it is the only book that asserts to describe who God is by telling the history of His chosen people and the revelation of His character through Jesus Christ. If we are to make sense of good and evil, we must put those ideas through the filter of God's character - and without the Bible, God's character becomes victim to our impressions (or worse, our desires) of Him. That leads to chaos. There must be a foundational "ground" to the identity of God, something objective in nature - otherwise, discussions of the character of God becomes meaningless.