It depends, States and local municipalities and jurisdictions had their own laws regarding segregation they varried. But I can say none of these "Jim Crow" laws decriminalized murder.

Let's say your in the deep south during this era of segregation, in a small back water town. Let's say an African American offended s white southerner. Would that southerner retaliate then and there? Most likely not. He might just let it go, or alternatively gather up some friends and mob mentality would take over. There were thousands of lynchings, often performed in public. Between 1882 and 1968 there were over 4700 documented examples of lynching [pp. 560-561] (not to mention the victims that were burned, beaten, castrated, or shot.)

What would happen if he did just murder an African American in broad day light? Tough to say as laws and more importantly people's attitudes towards racism and segregation varied so widely. Maybe the small town sheriff would "investigate" and chock it up to self defense? Maybe the victim's death caused some form of outcry and the perpatrator was arrested and tried in court. Then what, a prosecutor would need to convince 12 white jurors (some who may or may not hold strong racial bias) to convict a white man for the murder of a black man.

Assuming the perpetrator was arrested, the prosecutor did his job and the jury returns a guilty verdict. Then it's up to the judge to sentence (judges were not immune to holding their own racial bias that certainly had an influence on their sentences.)

So to answer your question it's tough to say. There are to many variables to give you a specific answer. I can guess, as a matter of conjecture that it was far more likely and safer for a white person to incite a mob to retaliate against an African American than to single-handedly exact revenge.

Boskin, J. (1976). Urban racial violence in the twentieth century (2nd ed.). Beverly Hills, CA: Glencoe Press.