You know what humans love to do? Other than "Dance Dance Revolution", we mean. We like to reduce our opponents straw caricatures or even tackle dummies whom we can easily destroy.
And that, Shmoopers, is essentially what Mark does with the Jewish leaders, who are very flat characters in this gospel. Here's why they exist:
• To oppose Jesus, giving him someone to refute.
• To secure Jesus's arrest and execution.
• To bear the blame for the crucifixion (see "The Parable of the Wicked Tenants").
This last one this has had a sad result for Jewish-Christian relations throughout history—but that's a story for another time.
But wait! There are a few important exceptions to this rule. Mark doesn't condemn people simply because they belong to one of the leading Jewish groups. Think about the synagogue leader Jairus, whose girl Jesus raises from the dead; the scribe in Jerusalem (5:21-24, 35-43), whom Jesus affirms as not far from God's kingdom (12:28-34); and Joseph, a distinguished member of the council, who provides for Jesus's burial (14:42-46).