I think by the time we get to Saul in BB, Chuck is almost certainly dead. Jimmy is too in awe of his brother to let him down by going all shady, so by the time he's Saul his moral compass has gone and he is out for what he can get. He realises that he was always the loser when he was Mr Nice Guy and the only person he could measure himself against is gone. I think the class action suit and Chuck's part in it is a major factor in what might close the season in a shocking way. Also, BCS isn't just about Jimmy's journey; it is also about Mike's, who is much more sympathetic in this, when he was a hard-nosed bastard in BB. He is just a moderately dirty retired cop here, but when we get to BB he is a pitiless killer, so something must have changed him in that time. When he killed those cops you could justify it as revenge, but later on he would kill anyone he was told to.
I am probably completely wrong. One of the strengths of this show is that you can't take much for granted and you don't have a clear read on what is going to happen from what episode to the next.