I think it's fair to say that a lot of romance fiction is powered by the idea of a frightening man, even if, as you say, he has a good reason. I admit that this conclusion is the result of realizing that I don't like the genre, and I think that's the reason.

Given that there are so many subgenres of romance, I suspect we are talking about different ones. In the small sample of my wife's books that I've read, the hero is never described as frightening to the heroine. Typically, he takes the form of an annoying rival who the heroine believes is overconfident or arrogant, someone whose goals are (superficially and initially) at odds with those of the heroine. (It then usually turns out that one or both characters have been operating on the basis of a mistaken impression about the other's goals or character.)

But I have never seen fear described as a heroine's reaction to anything except the villain, or her feelings for the hero. (Or more precisely, her anticipation of the problematic consequences of allowing her feelings for him to develop and be acted upon.)

Fear of the hero himself, or his actions, though? To my recollection, never happens in these genres.