Before reading this post, I encourage you to read my previous post on False Logic and the Myth of Moral Subjectivism, which will provide the requisite background for the following.
Another rebuttal from one of the atheists (more likely a non-theist, since no one can prove there is no God) is below.
It is true that we consider the Nazi's evil as well as Pol Pot. However, many people do not consider them evil and in fact, would call us evil. It depends upon what criteria one uses to determine what is evil or good. It also helps that we beat the Nazi's in the war. If they had won, the Allies would have been painted as the bad guys in the history books that the Nazi's wrote.
If your god was actually the basis for this, we should see that all people have the same foundation for making moral determinations. But we do not see this. Killing a is a good example. Most people consider killing to be bad. However, they will accept certain types of killing to be permissible. Mercy killings, killing for family honor and personal insults, to name a few, are perfectly acceptable reasons to kill for some cultures. Others would disagree.
Standards change over time as well. Slavery has been accepted in most cultures in the past. However, today, we find that concept to be completely unacceptable. Your bible even gives rules for owning slaves.
What this shows is that morality is not static, but fluid. It is very subjective.
I do not intend to be disrespectful with this reply, but the truth of the matter is that the only thing you have proven here is your lack of understanding of Scripture
For an atheist to decry anything whatsoever as "evil" or "good" is antithetical to the very epistemology (one's theory of knowledge) and metaphysic (one's view of reality -- where we come from, what is real, etc.) to which atheism professes. In a world of chance-driven subjectivism, all that exists is opinions and feelings -- both in the subjective sense. You may choose to call something evil. You may choose to eat carrots or chocolate or steak because you think they taste good. However, you have absolutely no objective moral basis for asserting that even a single other person should hold to your beliefs. This is one of the most audacious positions of atheists -- you (generally speaking) assert that beliefs such as Christianity are "wrong," "evil," "not good for society," “immoral,” or whatever is your choice pejorative of the day, while at the same time asserting that no objective morals exist by which to judge such things and make such statements.
Regarding differences in how mankind views and follows various moral values, Scripture does not state that all men will follow the Law written on their hearts in equal measure. It simply states that the Law is there and that we are commanded to follow it. Due to man's fallen nature -- a fallen nature that infects every aspect of a man's existence; the degree of which varies from person to person, from time to time -- we would naturally expect, and Scripture affirms, that man will not obey YHWH's Law perfectly. Far from this issue refuting Christianity, this fact, clearly stated in Scripture, is why we find various views on individual topics in our daily experiences. Man's sinfulness, when not brought into check by the Law and temporal judgment, has nearly no bounds.
Your point regarding the immorality of killing is also one of misunderstanding and/or a lack of wisdom. God gives clear Law when it comes to the taking of human life (even this distinctive of human -- not animal -- life is pertinent, due to some comments above[this was relevant to other comments not seen in this blog post]). Killing itself is not strictly forbidden in every instance. The death penalty is stated as how certain crimes are to be punished; wars where YHWH vindicates His people also involve Divinely sanctioned killing; etc. Of course, one of immature understanding may turn to the 6th Commandment in a vain attempt to show a contradiction. One who cares to take the time to study such things will quickly find that the 6th Commandment deals with murder, not killing in general. For an excellent treatment on the use of the words related to "kill" in the Old Testament, see Milton Terry's "Biblical Hermeneutics" (pp. 192-193 http://bit.ly/TerryBiblicalHermeneutics).
Yes, YHWH's Word does give specific laws that relate to the treatment of slaves. His Word also gives us specific instructions on how we are to proceed with divorces as well. However, again, wisdom requires deeper thought into this matter (though some things in Scripture are blatantly obvious, others require study and diligence...along with the illumination of the Holy Spirit). Malachi 2:16 clearly states that YHWH "hate[s] divorce." But then both Christ and Paul give instructions on how we are to go about proceeding with a divorce. Oh, the contradiction! Hardly. Though YHWH hates divorce, He is also merciful and gives us specific instances where we may be released from a marriage, because He knows that our sinfulness will inevitably lead to ungodly unions sometimes that should not be continued.
In similar manner, YHWH also attends to the protection of slaves by ensuring their masters have Law that gives clear instruction on how such slaves are to be treated. Another parallel to this is the fact that, in the beginning, YHWH made marriage to be between one man and one woman. Both Matthew 19:4-5 and 1 Corinthians 7:1-2 clearly make this point (…and so as to not be negligent, Gen. 1:26-28, as well). However, in Old Testament times, men were frequently polygamous. Did YHWY endorse polygamy? No. At that time, He simply did not choose to judge it temporally. Such was the case with slavery.
Finally, you state, "What this shows is that morality is not static, but fluid. It is very subjective." False. On the contrary, morality itself is static. It was defined by YHWH Himself in Scripture. Man simply suppresses the truth of Christ's Word to bend it to his every whim in an attempt to justify his sins (see Romans 1:18-32 for a clear Scriptural view of this topic). It is not morals that are subjective. It is man's implementation and application of those objective morals that is subjective.
For a continuation of this post, please see Part 3: The Myth of Moral Subjectivism.
Soli Deo Gloria!