Actually slander even refers to truth. There are times that one must say negative things but that is to warn someone. Otherwise even true fact are slander. [There are a few verses that refer to this subject. from different angles. One thing is "Rekilut" saying true things [even positive things] that can bring hatred between one person and another. [Like praising General Grant in front of General Lee.] Another thing is "Lashon Hara" which is saying negative things--even if true unless to warn someone.
[So why is true lashon hara forbidden? To R. Yona of Grondi [author of Shaarai Teshuva] is because of collateral damage. That is: the person might be subject to punishment that otherwise he would not get according to the Law of Moses. An example from the Gemara itself is testifying alone in a court of law. Since he is alone his testimony can not be accepted and so it is lashon hara. Some Rishonim disagree and hold that true lashon hara is forbidden in and of itself. Only in specific cases of the need to warn others is it permitted
[It is somewhat of a shock to me to see that most people do not even know that lashon hara is forbidden.