In Huxley's Brave New World, in what ways does Bernard Marx abuse power?

Bernard Marx abuses power by hurting others as he selfishly seeks popularity. He is an Alpha, which means that he is at the top of the caste system and is highly intelligent. As a result, he treats those in the lower castes as if they were slaves. For example, he yells at Gammas and Epsilons while disregarding their lives as servants. He is rude and does not value their lives. Marx also abuses his power when he ruins his boss's life and jeopardizes his friendships with John the Savage and Helmholtz Watson.
First, Marx ruins the Director's life when he brings back Linda and John from the reservation. Because Marx learns that his boss fathered a child years earlier--and left the pregnant woman at the Indian reservation--he has the power to bring them back to humiliate the Director. Marx does this because his boss threatened to send him to Iceland before he went to the reservation on holiday with Lenina. As a result, Marx abuses power when he exploits Linda and John in order to destroy his boss's life. 
Next, Marx gains prestige and popularity for introducing John into civilized society. Because John is vulnerable to the culture of this "brave new world," Marx retains a certain amount of power over and responsibility for the Savage. However, rather than help John assimilate into London society, Marx uses him as bait to get girls to sleep with him. Not only does he take John for granted, but he also uses women's interest in the Savage to exploit them. The text describes Marx's abuse of power in friendships as follows:

"Bernard continued perversely to nourish, along with his quite genuine affection, a secret grievance against the Savage, to meditate a campaign of small revenges to be wreaked upon him . . . As a victim, the Savage possessed, for Bernard, this enormous superiority over the others: that he was accessible" (179).

Finally, Marx's newfound power makes him feel as though he can toss his friend Helmholtz Watson aside for a new life. Watson is also considered "Bernard's other victim-friend" (179). When Marx's popularity falls apart, he goes back to Watson to ask for their friendship to be reinstated. Since Watson isn't vindictive, he takes Marx back as his friend.
In the end, Mustapha Mond doesn't let Marx get away with his "little revenges" on anyone--least of all in society. Marx is sent to an island where he will feel equal to others like him. This will help him not to feel so superior and powerful over friends or those in lower castes.