In Canada where I grew up we called them malicious gossipers.
This meant (to us) that even a kernel of truth was distorted by the person. Other words we'd have used would be rat bastard or nasty bitch, but they would have had to be in context. Slanderer and scandalmonger might fit, but the listener might not know the gossip to be lies.
Defamer and slanderer both fit, but aren't common here in everyday speech. A lawyer can help use sue for slander.
n. oral defamation, in which someone tells one or more persons an untruth about another which untruth will harm the reputation of the person defamed. Slander is a civil wrong (tort) and can be the basis for a lawsuit.
This still doesn't work for willfully distorting a message. That person is most likely a narcissistic liar.