Saul was a dirt bag in "BB" but he was never evil. He always tried to give Jesse and Walt good advice above and beyond being a criminal lawyer. In season 3 he tried to give Jesse advice on laundering his money, in season 4 he tried to convince Jesse to mend things with Andrea/Brock, in season 5A be obviously felt bad about helping Walt poison Brock, and wanted to end their partnership....even when Walt threatened him, he tried to talk him out of dealing drugs, despite the fact he'd lose money. There's also a Blu-Ray/DVD only scene for season 5A that takes place between the last two episodes....the gun Jesse had in "Gliding Over All" came from Saul, who was worried about Jesse.
Hell, the last scene he has in "BB" he's trying to talk Walt into turning himself in to help his family.
The first scene he has in "BB" he's already working with the cartel, so a lot has to happen for him to go from Jimmy, to Saul.
Fred.