It's no exaggeration to say that the sweet, mild-mannered teacher Katherine Barlow is turned into a notorious outlaw by racism. In those days, racial mixing was considered taboo. Black people were widely regarded as inferior to white people, so any hint of a sexual relationship between the races was regarded as a dilution of the supposedly pure white race.
Such patent nonsense may be ridiculous, but it had serious consequences. Many African American men were summarily lynched by mobs of angry white people simply because they were suspected of looking at a white woman in a certain way.
Kate's boyfriend Sam is just one of many such victims of racism and white supremacy. He is lynched because someone saw him kissing Kate. In this part of the world, the mere act of a Black man kissing a white woman is considered not just unacceptable, but dangerous, a threat to the stability of the whole community. And so Sam is punished, in the time-honored fashion, by being hanged.
In response to this outrage, Kate vows revenge. Before long, she's transformed herself from a small-town teacher who wouldn't say boo to a goose into Kissin' Kate Barlow, one of the most feared and dangerous outlaws in the West.