Because some people are those type of people they pretend to be you best friend one min then the next they are talking about you to a person who don't like you and you don't like them saying bad things about you and it makes you so mad that you just want to beat the crap outta them bit you can't mostly cause you are at school or work. But it really ticks you off.
On your case, your best friend talks about you badly right in front of you shouldn't be called as backstabber. I think it's better for your friend to talk in front of you rather than behind your back. Maybe he/she wants you to know how he feels about you and want you to look at yourself. Or maybe he's just at a bad mood that he didn't realize talking about it. It can't be called as a backstabber. You will not know what he was talking about if it's a backstabber.