Is there a word/term for a person who's sweet words are actually venom?

A viper? A snake?

Passive aggressive.

I’m pretty sure there isn’t anything in the DSM-IV regarding someone whose words are poisonous.

Do sirens count? They lured sailors into danger with their captivating voices.

Perhaps you’re thinking about the phrase speaks with a forked tongue as in, ”...to say one thing and mean another or, to be hypocritical, or act in a duplicitous manner.”

My guess would be that speaking with honeyed words concealing hate isn’t a diagnosable medical condition. It’s a behavior that can be described, as you have done, but probably not in a single word or technical term.

Pretty Woman.

Sugar wouldn’t melt in her mouth or was it butter?

Some have said galileogirl

sly, cunning, guileful

sociopath

Fucker.

You can build your own, and provided that enough people understand what you mean, you could use it.

@tinyfaery Doesn’t that have a more popular meaning
@plethora I think that refers to behavior rather than words. Remember sticks & stones etc
@sndfreQ Not unless it is a movie Native American speaking
@Sarcasm Very appropriate for this venue

A bitch, acid tongued bitch.

@faye You rang? or is that a misogynist in your pocket?

Politicians

Silver-tongued or Kaa

@billio check the link.

Removed by me.

Smarmy. (insincere and obsequious)

Duplicitous comes pretty close—exhibiting dishonest behavior that is intended to make somebody believe something which is not true. Two faced.

It’s not a single word, but silver-tongued devil is the archetype.

@galileogirl no misogynist, I’m female. I was thinking of my ex-mother-in-law!!

A silver-tongued devil.

Smooth-talker, smooth-tongued, a smooth-operator.

@Zen_Again – Now your just ripping off Sade.

Those were the days. What a babe – what a song.

@BluRhino @faye I like yours best, it insists on the dark side of the matter
@liminal I can’t follow your link liminal, unless I’m missing something
Thank you

what is a ” womanizer” ? for 500.

Yes, manipulative.

@12_func_multi_too http://www.youtube.com/watch?v=-T0I5UepXMA

ahaha!

@liminal
Kaa is working awfully hard there. Too hard.