Do you not understand?
Only mages seek power!
Lambert didn't seek to usurp the Divine or Chantry though.
He in fact even wanted the Chantry to be a part of the Circle system again once he was done putting down the Rebellion, he just didn't care for the Divine's actions, stance or her personally.
His actions were done out of belief, not lust for power.
If he had been after power, he likely would have turned the army he had assembled against the Divine, he even went so far as to call her unworthy to sit on that throne, but he didn't move to strike against her or the institution he was pledged to formerly.
Not a black and white character certainly but not an overt maniac either, he had his beliefs and he acted accordingly.