Sentence:  fearful of being mistaken for a Jew , he seeks protection in his Nazi uniform ; clinging to the enemy Jew idea , he is forced to overcome habits of politeness and neighborliness ; once in power he begins to give vent to a criminal <WSD>opportunism</WSD> that causes him to alternate between megalomania and envy of those above him . 
Meaning of the word marked with <WSD> tags:  taking advantage of opportunities without regard for the consequences for others