Look, it’s just this simple; men of good conscience cannot abide evil, they just can’t. Where evil exists, therein lies their purpose. Point blank, end of story. It’s just that simple.
There is no such thing as a “bad hero”, a little misguided maybe, but there ultimate selfless act made them persons who valued others more than themselves. A person like that is naturally drawn to evil. Evil, in the form of harming others, stirs the hero’s outrage like nothing else. Like moths to a flame.
It’s just that simple.