Being two-faced can be intentional and unintentional, innocent or malicious. Just part of someone's personality.

There are people who want to be two-faced to cause trouble, whilst others do it to make someone feel good about themselves and keep them as a friend. Then there's the gossip aspect. As my ex once said "you've two ears but only one mouth, use it wisely". I'm always careful what I say until I know a person, even then I am still wary. Doesn't just apply to friends, family too. My aunt thinks it's fine to spread gossip. She criticises me for not being open, but it's not my style. Personally I find it too much like hard work to keep two personas going. If criticising someone, I find the right tone so they don't feel worthless.