Can the God of the Old Testament be seen as a consistent moral law giver? There seems to be instances when God tolerates sin, since it is an improvement over the cultural conditions of the time. For instance, the laws regarding the treatment of slaves some have argued were encouraging people to treat slaves as human, and thereby eventually leading to the world we have today, in large part, which sees slavery as a great moral crime. Or requiring a rapist to marry the woman he raped is progressive. The argument goes that during this time in history, if a woman was raped she was essentially soiled for life; no one would want to marry her afterwards. This law is essentially forcing the rapist to take responsibility for his crime, and see that the woman is taken care of (materially at least, or in some sense, though it is hard to imagine today).

There are other times where God is incapable of tolerating sin, there are examples of the death penalty being used in punishment of transgression of the Sabbath, keeping spoils of war, striking your parents, etc.

How then can God be seen as a consistent moral law giver given these seeming contradictions?

Edit
For context, this is a question that I hear a lot from atheists as to why it is not rational to believe Christianity (or Judaism) in a particular. It is one I have a hard time answering. I am wondering what the Christian reply is.
I wouldn't have answered this had you not added the context.  However, with the context added....

It sounds like what you're looking for is an answer that will sound reasonable to an atheist that's asking this question.  As a former atheist and anti-theist, who thought that the Christian God was not only made-up, but that the (Please forgive me, Lord) "God that these Christians made up" was cruel, evil, and unjust, I'm sorry to say that there is no argument that can be given to someone with this mindset.

There are some things that God has commanded that are very hard to understand without the proper understanding of God's nature.  Understanding God's nature, and his view of sin is necessary to answer questions, such as "How could a moral God command the Israelites to kill all of the inhabitants of the various lands they took in Deuteronomy?".

To answer these questions alone sound hollow.  The answers are completely unsatisfying to an atheist that doesn't even believe that God exists, much less is wiser than us. 

For  samples question and Christian answer, see this link: http://www.rationalchristianity.net/genocide.html

The first question and answer:


  Why were the cities destroyed?
  
  The primary reason was punishment for wrongdoing. The populations of
  the destroyed cities had long histories of grievous sins (Gen 15:16,
  Dt 25:17-19), which often included sacrificing their children to false
  gods (Dt 12:29-31). Their consciences should have told these people
  they were doing wrong. Had they listened and changed their ways, they
  would not have been destroyed. God has said that if any nation is
  about to be destroyed as punishment but repents, he will forgive them
  and not destroy them (Jer 18:7-8). In fact, this occurred in the city
  of Ninevah (Jonah 3:4-10).
  
  In the cities that were given to the Israelites as their inheritance,
  there was a secondary reason: totally depraved cultures were destroyed
  so that they would not corrupt the Israelites into committing the same
  evil acts (Dt 7:1-4, 20:16-18). This did in fact occur: when the
  Israelites didn't obey God and destroy cities, they too began
  practicing child sacrifice (Ps 106:34-40).
  
  Additionally, the destruction of wicked nations served as an
  instructive warning to contemporaries (Josh 2:1-11) and future
  generations (1 Cor 10:1-11).


The above answer is completely unsatisfying to an atheist, who believes that the God of the Bible is made up.  In order for the explanation to sound reasonable, we ahve to first accept that 


God exists
He is the God described in the Bible
He knows better than us
He is righteous and incapable of doing wrong, therefore his actions were righteous.


An atheist, particularly, and anti-theist believes none of that.  Pretty much every answer to the questions about God's apparent "immorality" are based on an initial assumption.  That God exists and is good.   That becomes  circular reasoning, and an atheist can recognize it.  To satisfy an atheist, you would have to first convince them that God exists and that He is the God of the Bible.

Personally, when confronted with this type of question while witnessing, I answer honestly and say, "If you can accept that God might exist, I can answer this question, and it probably won't satisfy you.  If you believe firmly that He doesn't exist, then nothing I can say will answer this for you..."  and move on to something else.