"Mental disorder" is a matter of perspective, a cultural one.
In fact, the notion of "opposite gender" is also a questionable one, "opposite" being a tad extreme. More like the 'other' gender, as actual differences are few, usually in degrees as opposed to abslolutes, and rather maleable.
The need to bully people is a mental disorder that comes from someone getting beat down one too many times by daddy or their older sister or some crap, and as evidenced by the shitter being a jerk as a result, not beat down hard enough.
Relatively mentally stable folks do not bully people, they have no need to. They are cogent that doing so won't change someone from being a little bitch who couldn't defend themselves at home and are now taking it out on what very few they can find that are even more vulnerable than their bitch assed selves.
The only good thing about those who bully is knowing that no matter how hard they try, they will always know how weak they are, and once school ends, someone out in the real world will come along one day and remind them of that for good. Real hard. Real good.