I read the entire article :)
How much did you read? It wasn't even the alleged murder that incensed the mob, but the dubious claim of rape:
"But Deeter’s death wasn’t the principal outrage that wagged on everyone’s tongues: His companion, Mary Ball, accused Shipp, Smith, and Cameron of rape (an allegation she’d later recant). It was this — the story of Ball’s alleged assault told over and over — that incensed the white residents of Marion and surrounding towns."
The claim and fear that black men rape white women is a common theme of racist violence in America, historically and today. Supposedly the Charleston shooter said "I have to do it. You rape our women and you’re taking over our country. And you have to go." (http://www.huffingtonpost.com/2015/06/18/white-female-sexual...)
The victims of the lynching were guilty of being black, and that is why they were attacked. Although they probably wouldn't have had much luck in the racist criminal justice system, ostensibly they should have been innocent until proven guilty by the legal system, not assumed guilty by a racist lynch mob.