The Bible is a self-authenticating library of 66 internally consistent books. To use passages of the Bible to interpret other passages of the Bible is the foundation of proper Scriptural exegesis.
Any system of morality not based on God’s unchanging laws is merely someone’s opinion-of-the-moment that says, for example, murder is not ok under this circumstance but is ok under the other circumstance. If there is no absolute measure of right and wrong then there is no rational basis for condemning anyone’s behavior. Justice is then decided by "might makes right". The majority of our leaders in DC are criminals by my standard but they see me as a criminal if I refuse to obey their unconstitutional laws. Who’s right? My standard is absolute and theirs is circumstantial.
The Greek words translated "fear and trembling" do not instruct the slaves to be afraid of their masters. Rather, they are to have respect for their masters based on the institution of the authority that is placed over them, an authority that is an archetype of the authority God has over man. In this regard, "fear and trembling" is an idiom that is also used in 2 Corinthians 7:15 and Philippians 2:12 where believers are told to have such respect for God.
The instructions in 1 Peter 2:18 are directed at believers only and for the purpose of representing Christ even to unjust masters. I think your understanding of slavery in the Bible is skewed by our American history. Please read http://www.bible.gen.nz/amos/themes/slavery.htm. It is short and I think does a good job of explaining the vast differences between the two.
"And that's why I can mock a prophet of God and not get mauled." Actually, you can’t mock a prophet of God because they all died at least a couple millennia ago. As for the rest, requirements in the NT regarding behavior are directed at Christians only. That’s easy enough to prove because Hebrews 11:6 says, "And without faith it is impossible to please him, for whoever would draw near to God must believe that he exists and that he rewards those who seek him."
Ron Paul - Honorary Founding Father