What are the reasons that made Tony a completely asshole in the lasts seasons?

Currently watching the Sopranos and today I finished season 5. This was my favorite season, but the show has become much darker and more disturbing, especially in Tony's behaviors: hitting on Adriana, bullying Christopher telling him to drink and leaving his goomar after she got burned.

In general Tony seemed more aggressive and angry in every situation, treating his friends and relatives badly (when he saw that Janice seemed happy he tried to ruin the moment). I don't think that the Tony of the first seasons would have done all these things, what do you think this change in his personality comes from? I suppose some causes are the separation with Carmela and the tensions with New York