There are many synonyms (common informal nouns) which can be used to describe someone who talks badly about other people and who may cheat or lie. Here are a few of the synonyms (in order from bad to worst) gossip big mouth blabbermouth tattletale troublemaker liar backstabber backbiter fink rat snitch squealer stool pigeon

I'm not sure where the above commenter is from, but in the UK/USA etc 'rascal' and 'bugger' are not words commonly used - at least, not by young people. Traitor (noun) is the word that definitely makes most sense.. I know here in Ireland people say that such a person is 'sly' (adjective), and I'm pretty sure that is a word used commonly in the UK also - I am not sure about the USA as I have never lived there.

In this case, the word I would use would be "manipulative."

If the problem is that the person betrays their trust, http://www.merriam-webster.com/dictionary/tattletale among the synonims, snitch and rat in an urban environment.

People (the nouns describing people usually preceeded by "a/an") who do harm to other people, steal their ideas, ruin their plans, and do other immoral things may be called: cheats lowlifes thieves crooks scoundrels criminals turncoats traitors savages bad actors adulterers aggressor agitator animal antagonist bastard bigot brute bully bum cheater coward deadbeat deceiver degenerate delinquent deviate devil dog faker false-friend gangster hoodlum lawbreaker menace misfit money-grubber monster obstructionist opportunist outcast pervert pest phony pig predator profiteer protagonist psychopath punk savage scoundrel scum sinner slut snake stinker thief tormentor tough trash troublemaker two-timer user vandal weasel worm There are also many negative adjectives describing people's personalities.