"The original cause of the trouble was about twenty years in growing... Had you lived anywhere within fifty miles of Sundown Ranch you would have heard of it. It possessed a quantity of jet-black hair, a pair of extremely frank, deep-brown eyes and a laugh that rippled cross the prairie like the sound of a hidden brook. The name was Rosita McMullen..."
Though Rosita has many admirers, two stand out from the crowd. Madison Lane, a young cattleman, wins Rosita's hand and the two are married on Christmas Day. The spurned lover, Johnny McRoy, takes rejection hard, crashes the wedding, and shoots off his gun while yelling "I'll give you a Christmas present!".
"It was considered an improper act to shoot the bride and groom at the wedding" and the guests defend the couple. The shooter is deterred, but makes his intention known to "shoot better next time".
That night Johnny is reborn as the Frio Kid. The notorious 'bad man' is eventually responsible for eighteen deaths. Rosita fears for their lives, especially at Christmastime.
"Many tales are told along the border of his impudent courage and daring. But he was not one of the breed of desperadoes who have seasons of generosity and even of softness. They say he never had mercy on the object of his anger. Yet at this and every Christmastide it is well to give each one credit, if it can be done, for whatever speck of good he may have possessed. If the Frio Kid ever did a kindly act or felt a throb of generosity in his heart it was once at such a time and season, and this is the way it happened."
The Frio Kid does, indeed, have a Christmas gift for Rosita. It has been said that O. Henry is the master of surprise endings. He lives up to his reputation here, and I'm not going to spoil the story. You can read it for yourself here.