It's important to remember that these kinds of questions are usually not from someone who wishes to learn, but rather someone who wishes to tell you what they think to puff themselves up. God's thoughts are not our thoughts and His ways are not our ways. Not only are these kinds of questions ridiculous (especially since God is the author of morals and ethics), but they are arrogant, presumptuous, and very dangerous.
Rather than dealing with the specific issues, such as slavery, polygamy, war, etc. I would rather offer some important considerations regarding God's character, what the Bible says about it, and some guidelines to go by when facing these issues in scripture.
In regards to who or what God is it is clear from scripture that He is how the Westminster Larger Catechism describes Him in question #7:
"God is a Spirit, in and of himself infinite in being, glory, blessedness, and perfection; all-sufficient, eternal, unchangeable, incomprehensible, everywhere present, almighty, knowing all things, most wise, most holy, most just, most merciful and gracious, long-suffering, and abundant in goodness and truth."
Whether or not this is true is irrelevant to the person who "critiques" the Bible. If you are going to critique the Bible then you must critique its whole and accept what it says about who God is. Taking into account what the Bible says about God and His character you must enter into question with the premise that if God looks as if He is doing something unrighteous, then it must not be unrighteous. In fact, it must indeed be just and righteous and an act to carry out the greater good for those that love Him and are called according to His purpose.
It's too easy to take a passage which talks about killing a slave and judge God unrighteous and evil. Conversely, it is too easy to take a passage like that and simply say that God must have had a good reason for doing that. Where that is true in a simple and elementary way we must be prepared to give a more complete answer not only for the benefit of the questioner, but for our own benefit at understanding better the God we serve. God does not want blind servants and there are answers to these difficult questions.
God does not "condone" anything. For God to condone something would suggest He is not actively involved in the situation. That, of course, is not what scripture teaches us about God's involvement in every detail of life. I think there are two main ways that God works His providence:
1. God commands
2. God uses
This may be of a surprise, but God does not condone slavery- He, in fact, commands it! Man in his sinful state has taken this idea of slavery and perverted it and distorted man's thinking and behavior in it. When you study slavery in the Bible with proper hermeneutics and biblical presuppositions in place you will find that this is indeed true. What's more, any of man's perversions of God's ways He uses to carry out His perfect will- man means it for evil, but God means it for good. In fact, in His providence God may even look as if He is unrighteous for a time to carry out His purpose. Our lack of foreknowledge of this purpose is not unrighteousness on the part of God Himself, but rather it is the freedom of God and another example of His sovereignty and omniscience.
We must accept the fact that God exists, He is sovereign, He is holy, and His word is infallible and inerrant in order to properly handle difficult questions about topics which modern, natural man would see as totally and enequivicolly unrighteous. Natural man cannot see past his own ambition toward autonomy to properly understand the acts of the Creator he wishes to renounce.
Christians need not be afraid of these kinds of questions about God and His word because there are answers and we should not be afraid of those answers either. What we need is better understanding and technique in studying. God wants us to know Him and obey Him. We do both by learning and having an answer for the hope that lies within us.