A "rumor" is a doubtful truth, which means there is no proof. Circulating such things causes various opinions about the subject of the rumor. You are stepping at the right of the subject to explain himself and deny the accusation (if it's not really true). People get the wrong impression of the person as a result. You already judged him without knowing the truth first. You do not only affect the life of the accused person but as well as yours, you will have a reputation of circulating rumors uncontrollably. Thus, rumor-mongering is a bad practice.