[Elsie]

David was given permission to have many wives and he was deemed to be righteous by the Lord and by Nathan, the prophet at that time. It was only when David took on wives that weren't permitted by the Lord that it was wrong. From the Old Testament:

Uhhh... not exactly.

David took ANOTHER MAN'S WIFE.

[Utah Girl]

Yes, he did, after the Lord gave him permission through Nathan, the prophet at that time to have multiple wives. Not only that, it says in the OT,

8 And I gave thee thy master's house, and thy master's wives into thy bosom,..

That was David's downfall, coveting another man's wife, sending Uriah into battle so he would be killed and then taking Uriah's wife. He had so many other wives, but wanted that which he couldn't have.