All the white men need to do to "justify" their attack on the Berrys is say that Henry John was flirting with a white woman. This second-hand hearsay is the only reason that they appear to have for attacking the Berrys. Similarly, their later bragging and threats operate just as strongly as physical threats.
"They was waitin’ there for they gas when some white men come up messin’ with them—been drinkin’, you know. And Henrietta heard ’em say, ‘That’s the nigger Sallie Ann said was flirtin’ with her.’
Sheriff's reaction:
“And ain’t a thing gonna be done ’bout it,” said Mr. Lanier. “That’s what’s so terrible! When Henrietta went to the sheriff and told him what she’d seed, he called her a liar and sent her on home. Now I hear tells that some of them men that done it been ’round braggin’ ’bout it. Sayin’ they’d do it again if some other uppity nigger get out of line.”
In context, the only reason the men attack John Henry is because he is black. They don't need any other excuse because of their deep seeded racism, and their knowledge that there will not be any repurcussions... the law will look the other way.