I imagine one of the countless fights between brother and sister. The boy, brought up by the father to be the next powerful mafia boss, but weak, and coward in spirit. The sister, educated to be an obedient, unpretentious wife and mother, who instead goes away to find a love and a family for herself out of the obvious men's rules, and the less obvious Mafia's rules.
I imagine him, trying desperately to be a tough guy like Papà, and ending up again and again to pester his sister, to bend her will to his. He knows for sure women must obey to men. Just as Mamma obeys to Papà. He knows Papà and Mamma toss their heads when they talk about the daughter. Father would be pleased if it was him to teach her a lesson.
But she never listen to him. She, who had a son without being married, with someone not even involved with his father's mafia clan, even comes back at him, laughing at his sorry face.

Isn't it the oldest story in the world? When someone very stupid is also convinced to be very strong or powerful or righteous?
So he spent the night in the car near her place. And in the morning he got out, reached her door, called her down with an excuse. She climbed down the stairs, showed up at the front door where he shot her in the head.
Finally relieved of not having anymore to face that obvious example of his own weakness (his incomprehensible disobedient sister) he declares to the police: "I am proud of what I did."
It's Italia, 2006.

(or, maybe it's an entire different movie. They were lovers. She was his toy. But now, stronger with the newborn son and her new family, she was threatening to sell it all out, so that'd teach him to go around playing Mafia boss. This plot would make his murder motivated by slightly smarter reasons, and meaner. And his proudness would come out from entirely different regions.
Well, you pick your favorite movie. One is less likely than the other, but they're both plausible, in this country, today)