"They promised them freedom, but they themselves are slaves of corruption; for people are slaves to whatever masters them."
The purpose of 2 Peter is twofold: to strengthen faith in the imminent return of Jesus Christ and to warn against false teachers. This verse is part of the second purpose. The teachers are promising freedom, but they are really extending their own enslavement to things of the world. I think the point is most strongly made—for them and for us—a little later, at verse 21: "For it would have been better for them never to have known the way of righteousness, than after knowing it, to turn back."