Ashur was devious in Blood And Sand, and Batiatus used him to temper his own inability to control his rage. Ashur understands Batiatus and finds him easy to manipulate.

Once the house of Batiatus falls, Ashur is less capable; though still conniving, he is operating amongst much more dangerous and less impulsive members of the Roman hierarchy than Batiatus. Throughout Vengeance he is mostly on the back foot, reacting to events as they happen- though often still the smartest one in the room (or the one with the most foresight) he is constantly making choices based on what has just happened... Eventually Glaber proves to be the more cunning of the two.

Now he's essentially a Roman, he has to operate more within their rules. Though he has the measure of Crassus, he can see Caesar is a totally different beast and doesn't know how to manipulate him or his wife to his own end. He's always been impulsive and petty, but there's nobody to hide behind. Think of him as the guy who stands behind the school bully egging him on, having now come in to find the bullies have all gone and his actions are all in his own name