I'd rather Garrosh be "insane" than possessed.
A possessed or corrupted villain seems to be cliche for myself and others at this point. We've fought countless enemies who were corrupted by demonic energies or possessed by otherworldly powers.
I think villain who is aware of his or her actions and thinks rationally about the authorities they commit, who is in control of but makes the conscious decision to transgress is far more interesting in terms of characterization.