The God of Abraham, Isaac and Jacob can do no evil. He can do no sin. That is a part of His very nature. If having more than one wife is evil then Yahweh God helped David to sin. Yahweh spoke through Nathan the prophet to David in the following verse.
God Gave David His Wives
2 Samuel 12:8
"And I gave thee thy master's house, and thy master's wives into thy bosom, and gave thee the house of Israel and of Judah; and if that had been too little, I would moreover have given unto thee such and such things."
So here we have God telling David that he would have given David more wives if he had asked for them.
Oh, but that was for the Old Covenant some will say. This would take the discussion into the Law of God. There are several articles on that subject at Oracles of Yah.
Feel free to send your helpful comments.