There can be many different interpretations of Lee's intention in including this dialogue, but one possible explanation is that it makes the people who convict Tom Robinson stand out as individuals rather than as a mob, a tactic that Lee employs often in her examination of racism and bigotry through the eyes of an innocent narrator.
The line, "Guilty ... guilty ... guilty ... guilty," occurs when Judge Taylor polls the jury at the end of Tom Robinson's trial. In polling the jury, the judge must ask each of them as individuals to confirm that they agreed to the decision when it was made and still agree to it before the court. This forces them to confirm that they each individually believe Tom Robinson should be found guilty. In adding this dialogue, Lee is emphasizing the choice that each individual person must make to participate in racism.
Scout watches Jem hear the jury being polled, and notes:
His hands were white from gripping the balcony rail, and his shoulders jerked as if each "guilty" was a separate stab between them.
It is not the verdict as a whole that has the most effect on Jem but each individual juror's decision. The fact that these people could each make the choice to condemn Tom Robinson is deeply upsetting to Jem, and he declares later that "It ain't right." In addition, the fact that Scout never narrates the official announcement of the verdict, but only the polling of the jury, shows that she, too, is focused on the individual people rather than the collective.
This is a recurring theme in the novel, such as when she is faced with the mob coming to attack Tom Robinson at the jail and focuses on Mr. Cunningham, the familiar face in the crowd, rather than the whole group. As an innocent, Scout is unaware of the societal pressures and peer influences that maintain racist structures. Instead, she sees people making their own decisions and carrying out their own actions, and this is emphasized by Lee with the dialogue, "Guilty ... guilty ... guilty ... guilty" in the trial.