Commentaries:
Peter puts it plainly: Balaam was insane—he was totally willing to oppose God Himself. He was not insane in the sense that he needed to be locked in a mental facility, but he was insane in his willingness to fight the Maker and Ruler of the entire universe! Moreover, Balaam was not backed into fighting Him—he went seeking Him to get his own way. That is certainly insane and why Peter used the phrase, "the madness of the prophet."
Richard T. Ritenbaugh
Balaam and the End-Time Church (Part 2)
Other Forerunner Commentary entries containing 2 Peter 2:16:
Galatians 1:6