What is the source of slander and swear words (derogative words) why do people use them? how is it in Judaism?

ב"ה Shalom First of all for sake of clarity, let's differentiate between "slander" which in the Torah is called "Motzi Shem Ra" when a person speaks false negative things about another person with no justification. This type of slander is in the Torah and considered as a forbidden action. (See Ketubot 46a, Rambam Ne'ara Betula 3:1) When you are referring to , if I understand correctly, is what is called using profanities or "foul language " or "swearing" . In regard to this, there is no specific Torah prohibition, but is clearly forbidden according to the Rabbis in the Talmud. The Ramban (Nachmanides) in his commentary on the Torah to the beginning of Parshat Kedoshim says that the torah gives us a commandment to be holy. This commandment includes all types of moral behavior including "clean speech." The Ramban explain that the Torah did not detail every type of negative behavior and to say that certain immoral things are permitted since the Torah didn't specifically forbid them is considered being a "vile person under the auspices of the Torah" or in simple terms meaning totally hypocritical. The prophet Yeshayahu (9:16) speaks strongly against a "mouth speaking profanities. "The Talmud ( Shabbat 33a, Ketubot 8b) learns from this verse that one who speaks profanities, even if seventy good years were decreed upon him, it can all be reversed and changed to 70 bad years." The Ramchal in his book Mesilat Yesharim chapter 11, refers to profanities by using the harsh term "prostitution of the mouth". There is a lot more to be said about this subject and how the Torah refers to the purity of the mouth. All the best