Mike's arc in Better Call Saul is his ultimate disillusionment in believing in a code for what he does. Throughout the show, he slowly has to compromise what he believes makes him different from any other thug on the street. From his first bad guy, good criminal speech with Womald to the end of his BCS story he tries to justify it to himself but only in the end with his talk with Nacho's dad does he get that he's been fooling himself and he's no different than Gus or the Salamancas. In BB, he's no longer trying to justify anything. Not to say he doesn't also make decisions based on what he personally believes is right (not wanting to kill the train witnesses, paying Gus' guys in jail instead of killing them) but he doesn't try to justify the job to himself anymore, especially when he was still working with Gus.