I think there is another reason for this practice: Imagine that one criminal has good leadership skills and is able to manipulate another, weaker criminal. He

I think there is another reason for this practice: Imagine that one criminal has good leadership skills and is able to manipulate another, weaker criminal. He could then convince the weaker criminal to pull the trigger and escape consequences himself, and then do it again and again with different weak criminals. By sharing the blame the system compels the leader, whoever he is, to restrain the underlings rather than push them to the extreme.