One of the things that strikes me in Mark 15 is the level of influence the religious leaders had over the masses. When Pilate offered the people the chance to free Jesus or a murderer, the religious leaders convinced the crowd to have the murderer released.
I'm not sure how this happened, but it isn't the only time something like this has happened in the history of the world. Religious leaders can be very good at convincing people to do bad things. Modern dictators take this lesson and create their own "fake religion" to stir up followers into a religious fervor. (Think Hitler in Nazi Germany.)
As Christians, we need to beware of such things. Paul admonishes us to be sober minded in all things. I wonder if some sober minded people in the crowd could have talked down the mob mentality that spared a murderer at the expense of one who had no guilt. Perhaps not, but at least those sober minded individuals would not have guilt of their own.