How to deal with negative people

 In everyday work, we all come across people who are negative, cunning and manipulative. Many of them display their negativity upfront and some are sweet as sugar on the face and strike behind your back. While you can deal very well with the first kind of negativity by retorting back or remaining calm while the verbal assault is on, it is very difficult to deal with the second kind of negativity. When somebody speaks negatively behind your back, you are unaware of what the other person is saying. You take their sweet as sugar front to be true and trust them and all the while they are backstabbing you. 

Recently, I came across such an experience and when I faced negative feedback from my boss, I realized what this friend of mine was doing. I was trying to figure out a way of dealing with this and trying to understand why I was constantly feeling nervous and stressed. Why were negative thoughts crowding my mind?

Then on discussing it with a colleague I realized that my friend was the cause of all this. I also asked my colleague how to deal with the situation. He advised that I should also maintain a sweet as sugar façade and while I can't backstab, I can always remain alert as to how she would behave in future and stop me from trusting her. I accepted his advice as he had met the same treatment from her and decided to be cautious more than anything else. 

How do you deal with such a person? Do you expose them if you are able to or do you remain cautious or speak upfront and tell them you know their tactics and are not going to become a victim?