What the Bible says about Anger as Zeal
(From Forerunner Commentary)
The margin gives "anger" as a substitute for the word "heat." It would read, then, "the anger of my spirit." He is talking about a kind of zeal—a heat of one's spirit. Such a heat or anger is motivating! It is something that makes a person get up everyday and accomplish his work. It is a feeling from inside that a person cannot suppress. It must be expressed somehow. The prophet does this through his preaching.
Richard T. Ritenbaugh
The Two Witnesses (Part One)