What will you do if your friend insult you from behind?.

Sometime a friend who always act so good in front of you could also be a betrayal behind you,he/she might talk nice,support you,or even stand with you all the time but will also tell other people what’s so bad about you after that(when you’re not there),in the country where I live many people claim that this is a custom in people’s social life,whether or not it’s for your family or friend(and negative value about someone is necessary).
I don’t understand these people,are they do that for popularity?,for fun?,or it’s simply a bad habit?,but I know that they won’t feel guilty after doing that(some of my friends act like this),do you think there’s nothing wrong with this behavior?,what will you do if someone treat you like that?.