I know what the Scriptures teach about how we Christians today are to treat our fellow man and that it is a serious sin to murder, that being so, Is it true that during the Old Testament age the slave masters had the right to severely beat or to even kill their slaves?
The slave laws of the Old Testament were unusually kind to slaves when compared to those of surrounding countries. Slave owners were specifically commanded, "You shall not rule over him with rigor, but you shall fear your God" (Leviticus 25:43).
The law recognized two classes of slaves: Hebrew slaves and Gentile slaves.
Hebrew slaves could only be held by a master for six years. "If you buy a Hebrew servant, he shall serve six years; and in the seventh he shall go out free and pay nothing" (Exodus 21:7). In addition, every 50 years a celebration was held called the year of Jubilee. During that year all Hebrew slaves were freed regardless of how long they have served. "And you shall consecrate the fiftieth year, and proclaim liberty throughout all the land to all its inhabitants. It shall be a Jubilee for you; and each of you shall return to his possession, and each of you shall return to his family. ... And if one of your brethren who dwells by you becomes poor, and sells himself to you, you shall not compel him to serve as a slave. As a hired servant and a sojourner he shall be with you, and shall serve you until the Year of Jubilee. And then he shall depart from you-he and his children with him-and shall return to his own family. He shall return to the possession of his fathers" (Leviticus 25:10, 39-41).
A freed slave was given provisions as he returned to normal society. "If your brother, a Hebrew man, or a Hebrew woman, is sold to you and serves you six years, then in the seventh year you shall let him go free from you. And when you send him away free from you, you shall not let him go away empty-handed; you shall supply him liberally from your flock, from your threshing floor, and from your winepress. From what the LORD has blessed you with, you shall give to him" (Deuteronomy 15:12-14).
It might seem surprising, but Hebrew slaves were given the option of remaining with their current master permanently. Yet it makes sense. As a slave, they enjoyed many rights and were feed and clothed. For some people, such a life was preferable to attempting to make it own their own. "And if it happens that he says to you, 'I will not go away from you,' because he loves you and your house, since he prospers with you, then you shall take an awl and thrust it through his ear to the door, and he shall be your servant forever. Also to your female servant you shall do likewise" (Deuteronomy 15:16-17).
The main difference between a Hebrew slave and a Gentile slave is that the Gentile slave did not have the right of automatic freedom. However, other rights and protections remained.
In Israel, a runaway slave from another country could not be forced to return to his master. "You shall not give back to his master the slave who has escaped from his master to you. He may dwell with you in your midst, in the place which he chooses within one of your gates, where it seems best to him; you shall not oppress him" (Deuteronomy 23:15-16).
While an unruly slave may be beaten, he could not be put to death or permanently injured by the master. "And if a man beats his male or female servant with a rod, so that he dies under his hand, he shall surely be punished. Notwithstanding, if he remains alive a day or two, he shall not be punished; for he is his property" (Exodus 21:20-21). The punishment for death of an individual was death regardless whether the person was a slave or not, so this would restrain a master's hand. Further, if a person is permanently injured due to a beating, the slave is immediately freed. "If a man strikes the eye of his male or female servant, and destroys it, he shall let him go free for the sake of his eye. And if he knocks out the tooth of his male or female servant, he shall let him go free for the sake of his tooth" (Exodus 21:26-27). It is likely that this was in addition to the normal punishment of inflicting the same damage on the abuser (Deuteronomy 19:21). In other words, the law regarding surviving after a beating was to encourage thought about cause and effect. People die of old age, disease, and unsuspected ailments. So when a slave died, was it due to the beating or not? Given the lack of sophisticated analysis, the judge's decision was simplified. If a slave died shortly after a beating, the judge was to assume the master used excessive force and he was to be tried as a murder. If the slave did not receive a permanent injury and died several days after a beating, it would be assumed by the judge that the death was a coincident.