I reference the trope of the thug and the innocent in a post about The King and I. In M/M-Yaoi, this trope shows up as a far older thug enamored with a young innocent. The age gap sometimes upsets the appalled-by-everything-that-doesn't-match-21st-century-mores crowd.
I'll grant that occasionally, in manga, the sheer youth of the young innocent can be troubling. But I find a state of being appalled very tiring, so I don't bother. (I do wish Akihito would go back to looking 20 rather than 16, as he does in the latest volume of Finder.)
A fun variation on this trope is the older thug who is enamored with a young sweetie-pie, who appears innocent but is in fact rather scheming (behind that innocent face). The thug is left wondering, "How did I get into this?" The thug then usually decides, "Eh, whatever."
The thuggishness of the thug prevents us readers from feeling bad that he has now become the pawn of an utterly manipulative so-called angel. And the use of a M/M relationship prevents the knee-jerk screams of dominance. The reader is given time to perceive the relationship as slightly more complex.
Waru is the most classic example. Joe, the thug/bodyguard, is amused and not entirely in thrall to his young innocent. He also decides--when the young innocent finally gets the thug where he wants him--"Hey, this life is better than what I had!"
