Polygamy and murder are two different issues entirely.
Murder is, and always has been, wrong.
In the Old Testament, polygamy was not viewed the same way.
Jacob, with whom God made a covenant, was never criticized by God (or anyone else) for polygamy. Hannah, the mother of Samuel the Prophet was one of two wives. In her story we can see the tensions that this can cause – but neither she nor her husband was criticized by God. King David, known as “The Man After God’s Own Heart” had many wives – and the only one for which he was criticized WAS an actual act of adultery. Judgment came against King Solomon on account of his wives – not as to their number, but rather because of their idolatry.
Polygamy WAS specifically forbidden for church leaders by St. Paul.
However, in no sense, in biblical times (whether Old or New Testament) could polygamy be considered “adultery” in the traditional meaning of the word.
CS Lewis, in *Mere Christianity * suggests (and I paraphrase) that “People have argued as to how many wives one should have – but not that one can simply take any woman one wants!”
In other words, even where polygamy was accepted, there was an understanding between relationships which were legitimate as opposed to illegitimate.
The understanding that monogamy is a more properly ordered lifestyle than polygamy is an example of the growing and deepening of the natural law, whereas murder has always been rejected by the natural law.