Souvenirs
Kiyotaka Ishimaru keeps souvenirs of everyone he's killed. Weapons, clothing, jewelry, teeth, bones, medals, trophies, among other things, hang on his walls in the mansion he's saved up meticulously to buy. His castle, his throne, his crown jewel, Ishimaru is proud of himself. He made this through hard work. Not like his grandfather, who sailed through life on his genius and failed. And not his father, who gave up his possibilities and future for his mother, whom he impregnated out of wedlock. No, Ishimaru wouldn't let anyone, or anything, take this from him.
Had you asked him if he'd be willing to share it, to marry, he would refuse. Not only was it his, his possession (Omegas are naturally possessive, after all), no one had ever loved him, and he, as an assassin, could not love, after all.
For who could ever learn to love a killer?
Perhaps, maybe, another killer.
And so began our tale of crime and punishment.