The starting point for your answer is:
What reforms were the Decembrists trying to make?
How did the revolt threaten the power of the czar?
Why did the revolt fail?
The revolt, by mostly more junior officers reflected the need for political reform, which was a threat to the czar's control. The Decembrist revolt was a reflection of the growth of liberalism that was occurring at the time, which the autocratic leaders of the time, particularly in central and eastern Europe, were looking to crush because it was a threat to their power.
