Before returning to the Old Testament, perhaps we should consider a few more questions concerning the slavery issue. Does Paul’s remarks about Hagar in Galatians 4:30 really show he approves of the mistreatment of slaves – “Cast out the bondwoman” – as some critics are quick to claim? Does God really command, as a few assume, that his people exclude the children of slaves when considering the distribution of one’s goods? 
First of all, we need to consider the context of Paul’s remark in Galatians 4:30. Is he speaking of the subject of slavery as an issue by itself, or does he use the idea of slavery to make a point in his argument with the Galatians? I believe Paul sought to make a point about the Galatians chosen position by using the subject of slavery. Notice how he addresses the subject throughout his letter but especially in the fourth chapter.
Paul began speaking of bondage in Galatians 2. He claimed that he went up to Jerusalem for the second time in fourteen years to meet with the Apostles, and he brought Titus with him, but the Apostles never demanded that he be circumcised (Galatians 2:1-3). Nevertheless, there were some, whom Paul describes as false brethren, that sought to bring them into bondage – i.e. demanded that all be circumcised (Galatians 2:4). Nevertheless, Paul and those with him (including the Apostles) never submitted to their desire for a moment (Galatians 2:5). The next time Paul brings up the subject is in Galatians 3:28 where he claims all are equal before Christ, whether Jew or gentile, male or female, bond or free. So, Paul cannot be accused of impropriety toward those who are slaves. According to Paul all are brethren in Christ.
The next and final time Paul brings the subject up in his letter to the Galatians is in chapter four. Here Paul claims that during our tutorship (i.e. held in bondage to the elements of the world, sin) there was no difference between us and a slave. However, when Christ came he freed us from our bondage to the elements of this world to become an heir of the Kingdom of God (Galatians 4:1-7). Nevertheless, now that the Galatians were free and heirs of the Kingdom of God, by submitting to the men from James (Galatians 1:6-8), they showed that they were dependent upon the elements of this world again whereby they were returning to a life of bondage (Galatians 4:8-9).
Next, Paul used Abraham’s sons as an allegory to point out what the Galatians were doing with their freedom in Christ. He says that Abraham had two sons—one from a bondwoman and the other from a free woman (Galatians 4:22). The son of the bondwoman pictured a son of the flesh, while the son of the free woman was the son of promise (Galatians 4:23). Paul calls this an allegory in Galatians 4:24, which pointed to two covenants. Hagar and her son pointed to the covenant made at Sinai (the Old Covenant – see Galatians 4:24-25), which answers to the Jews who were under the Law at Jerusalem. However, Sarah and her child answered to the promises, the New Covenant or Jerusalem from above, the mother of us all (Galatians 4:26-29; cf. Revelations 12:1-17).
In this context we find Galatians 4:30 “Nevertheless what saith the Scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the free woman” (KJV). That is, cast out Sinai, those who trust in the Law of Moses, the Jews who then inhabited Jerusalem etc. Nevertheless, we—i.e. those of us who are neither Jew nor Greek, male or female, bond or free (Galatians 3:28)—are the children of the free woman, heavenly Jerusalem (Galatians 4:26, 31). Therefore, we need to stand fast in the liberty in which Christ has made us free (Galatians 5:1).
Considered in this context, the accusation that Paul advocates the mistreatment of slaves, or disposes of them as unworthy of the New Covenant, is nothing more than a sad joke. The idea is preposterous. Moreover, neither can Paul’s allegory be taken as anti-Semitic, since Paul was also a Jew and bemoaned the position his brethren after the flesh had chosen against Christ (cf. Romans 10:1-3). He even fought those brethren who opposed him with tears of mourning, wishing it could be otherwise (Philippians 3:1-2, 18).
 As I said HERE, this current theme about “making sense of the Old Testament God” is based upon the book: Is God a Moral Monster by Paul Copan. These are my thoughts about his book. He may or may not agree with the impression his book has made upon me, but I thoroughly enjoyed reading what Paul wrote and recommend his book to anyone who is looking for a good read concerning defending our faith.