It makes perfect sense to say you poisonous snake. You could also say ‘you’re venomous’ which is also in the context of a snake. Some other words to describe someone like this would be: deceitful, malicious, vindictive, manipulative, calculating, disloyal, malevolent, demonic You could call them a charletan or a scoundrel, degenerate

To describe a person who is snake-like, you can use the word "serpentine." It is not a very common word amongst speakers of English, but it literally means "of or like a serpent or snake." If you want to sound eloquent or poetic, it is a good word to use, but it might sound pretentious if you use it in everyday conversation. Hope that helps!

Actually, it should be venomous, not poisonous. If you eat something and it makes you sick, it is poisonous. If something tries to eat you and it makes you sick, it is venomous. Therefore snakes (some of them) are venomous. However, leaving that bit of pedantry aside, it is probably not a good idea to use that expression. The usual expression for someone who is treacherous is 'snake in the grass'. If speaking directly to them, then you can just call them a snake. The poisonous/venomous part isn't relevant here, as it isn't necessary for the person to get close enough to bit you to cause harm. It is their secretive slithering about largely unseen that does the damage -hence snake in the grass, which can be quite difficult to see.

I think you can and it made me giggle and I'm sure many English people have said that to each other. However I think it's best to not say anything :)