I believe terrorism is justified in many cases. I believe that if you refuse to try to come to an understanding with someone, and instead force that person to behave according to your naive views, then any degree of force is justified to stop you and all who support your doing so. I also believe there is no way to differentiate this from "overthrowing tyranny" from an objective perspective. The only way to solve the problem is for the 2 sides to come to an agreement willingly (even if its just 1 person).
I also believe it is up to the person being forced to accept a view to decide what balance between trying to cause someone to understand your need for them to follow their views (when it really is necessary like regarding murder or something) and severity of the action used to try and force you to do so warrants violence. For instance someone might follow a law and decide to try and change it through politics if they truly believe the system is capable of being fair.
I also believe that the more deadly the single person has the potential to become, the safer the individual is from a government.
Some various counter arguments that I feel are wrong:
That terrorism attacks the wrong people: The people on the streets of the us are not innocent, they fund the governments which act in ways that motivate terrorists and they allow such actions to continue. This type of thinking is recognized by governments all the time: examples: uneasiness with countries trying to be neutral in major wars, invasions of countries like afghanistan which merely harbour terrorists etc. The truth is people in US have too high of an opinon of themselves and are too unfamiliar with death thinking it is something that "only happens to non-verbal supporting characters". We sit in the relatively comfortable USA while we send soldiers who accelerate every day violence to the point where survival is difficult. We may have earned this relative safety within the confines of the US, but our will to close ourselves off (allowing 3rd world countries to exist in parallel with ours) is ultimately responsible for violence pouring over to the us. By this I mean basically, you are starving and your neighbor has enough food for 2 but refuses to share, you are going to do what you have to do.
That terrorism is too unanticipated and doesn't give people time to react to your claims without bloodshed. The US government has counterargued this reasoning forever, claiming that ignorance of the law is no excuse. Random people are often made examples of, all of course who "didn't know" it was that serious or didn't know they could get caught (which if you accept that the human sense of morality is a sense of power, you realize are one and the same)