There is a word to call this kind of person:
- habitually say bad things about other people, ONLY when the said people are not present
- always excited to spread rumors, especially the bad things about some people he/she does not like
The word is something like "long tongue" or something like that, but I just cannot recall.
One who spreads malicious gossip.
or:
Someone who makes a false and malicious statement or report about someone.
to talk spitefully about (an absent person)
And the person is a backbiter.
Backbiting may occur as a form of release after a confrontation. By insulting the opposing person, the backbiter diminishes them and, by doing so, restores their own self-esteem.
Perhaps you just mean gossiping? Or some variation like tattletale?
Political correctness apart, such a person can be called 'a denigrator', from the verb 'to denigrate', from Latin 'denigrare' from 'niger', 'black'. So, 'denigrating someone or something' is 'painting it or them black'… that is, blacker than they are, which I could not find in a dictionary as an idiom, but I found the following related idiom in the Longman Dictionary of Contemporary English
not be as black as you are painted not to be as bad as people say you are
However, 'denigrating someone' can be done in the person's presence as well as behind their back, whereas 'gossiping' is strictly behind people's backs.
You could say that person has a spiteful tongue.
spiteful: spiteful implies a deep-seated malice or malevolence provoked by a desire to get even with others for real or fancied offenses; it suggests meanness or venomousness and refers more often to utterances than to acts: spiteful gossip; she has a spiteful tongue
Alternately, consider catty.
catty: spiteful, mean, malicious
She's catty about people.
traduce - Speak badly of or tell lies about (someone) so as to damage their reputation.
defame, malign, slander, blacken someone's name, talk smack, badmouth.
