While this question cannot be answered objectively, I would argue that violent political action is never justifiable in a democracy. I cannot prove that this is so, but it is my view.
In a democracy, people have many chances to express their political points of view. They have essentially unlimited freedom of speech. They have the right to vote. They can try as hard as they would like to convince others of the rightness of their ideas. Therefore, if the government is acting in a way that they do not like (or is failing to act in a way that they do like), they do not have the right to act violently. The activists have clearly failed to convince their fellow citizens. A person who cannot convince others to agree with him is not justified in engaging in violence as an alternative to persuasion.
In a true democracy, there is no justification for violent political action. Violence is only warranted when the government or some group of people is acting violently towards a group of people. This should not happen in a democracy.