Victims accused of crime
Sometimes victims of lynchings were accused of serious crimes but were killed without proper trial. That means we can't be sure if they actually did any of the crimes they died for. All too often victims were accused of minor crimes like shop lifting or had simply gone against local customs.
The issue of the victim's guilt is usually secondary, since the mob serves as prosecutor, judge, jury, and executioner. Due process yields to momentary passions and expedient objectives. 
Victims not guilty of crime
Originally whites used lynching to punish black slaves but whites who were brave enough to protest against this also risked getting lynched. After 1867 when the Ku Klux Klan were formed lynchings of black people became far more common. Sometimes the KKK have lynched people for being black and influential or for opposing Segregation or simply for being black. Asian immigrants, European newcomers, Jews, Latinos and Native Americans were also sometimes targetted. 
"3,724 people were lynched in the United States from 1889 through to 1930. Over four-fifths of these were Negroes, less than one-sixth of whom were accused of rape. Practically all of the lynchers were native whites. The fact that a number of the victims were tortured, mutilated, dragged, or burned suggests the presence of sadistic tendencies among the lynchers. Of the tens of thousands of lynchers and onlookers, only 49 were indicted and only 4 have been sentenced." 
Far too often the police did no proper investigation of lynchings and allowed the evil to carry on with impunity. 
- Strange Fruit: A protest song with enduring relevance This tells about a protest song about cruelty and murder involved in lynching.