I don't know- I'll be honest.

From the information you've provided, it just sounds like he's a jerk and not very nice to you and does whatever he wants, even talking trash about you- and you try to come up with any reason you can to excuse him.

That's a natural defense.
But not everyone has clinical issues that they need to work through, as was suggested.

Some people are just jerks.
They just use people, do what they want.
They simply lie to get what they want.
They aren't passive chameleons, they are active manipulators.

Obviously, just one possible scenario, but maybe one you should consider.

I'm not certain there is an acceptable reason, for ME anyway, that my fiance could provide to me why he would talk crap about me to his friends and then treat me well when around me.

You likely have a different perspective as you actually know the situation and details intimately.

I don't know your fiance or you, but maybe try to step out of the situation and look at it from another angle?