Discussion centered around the seeming difference between the Old and New Testament, especially where it pertains to certain obscure things, are resolve very easily by remembering the following:
1. The Bible is a PROGRESSIVE REVELATION! Men could not receive all of the truth about God or His ways because they were hardened in sin and needed a new birth. This would only come through Jesus.
2. God ACCOMMODATED fallen men's fallen state until Jesus came to bring them back to God's original intent.
In Matt. 19 Jesus explains that the reason for "easy divorce" in the Old Testament was because God accommodated their fallen state. Jesus came to show them that the proper way is to work through marriage difficulties.
You can apply this to laws of slavery, war, etc. These were accommodations due to hardness of heart. Heck, there was very little even said about Satan during that time since the people were so prone to idolatry that they may have started worshipping him if he had too much exposure. It is only when men could handle that there was an opposing force against God that Satan could be revealed without us worshipping him as a separate deity. It is also the reason why God took responsibility for a lot of the things that Satan did (2 Sam. 24:1; 1 Chron. 21:1; Job 1-2).
The NEW TESTAMENT is simply part of a progressive restoration to God's original plan for man.