When is an attack a terrorist attack?

Well, they'd still be labelled a terrorist. Firstly, because they are a non-state actor. Secondly, they are targeting the population, not the government apparatus.

Well, they'd be labeled as terrorists by the people being attacked, the people who agree with them on the other hand would probably feel justified in calling them freedom fighters, even though they are using terrorist tactics.

Do you think that a state can never be considered a terrorist?

Definitely, have you seen the opening scenes of Munich? When the members of Black September successfully capture the Israeli athletes, there is a stark contrast in the reactions between the two nationalities. The people living in Palestinian territories are seen to be cheering while the people in Israel are seen to be weeping and concerned. But if we're making the debate on terrorism as objective as possible, it should be measurable. That means a subjective interpretation need not be required and what we do is measure it against the tactics. And the tactics in this case are the target-set: Are the people targeted part of the government apparatus, involved directly in policy-formulation or part of the nation's security apparatus?

As for state's being terrorists themselves, in my view, they can't. This is mainly derived from the internationally recognized definition of terrorism that Bruce Hoffman asserts, that is, one of the prerequisites for a terrorist is that they are a non-state actor. This mainly stems from that states possess a monopoly of force and they are the only actors able to use it. That doesn't mean that states can use force unconditionally without consequence but it distinguishes themselves from being a terrorist.

freedom fighternoun
a person who takes part in a violent struggle to achieve a political goal, esp. in order to overthrow their government.

I haven't seen it, but I can well imagine.

Couldn't they fit both labels simultaneously? According to my dictionary:

I don't see anything incompatible with terrorist tactics there...

I see what you mean there, but it seems like a bit of a technicality. If a state is using terrorist tactics, I feel they should be considered the same.

What exactly does "six of one and half a dozen of the other" mean with regards to Bloody Sunday?

The thing about dictionary definitions is that they offer the most simplistic definition to satisfy all readers. But, even in that definition, while it is true, does not take into account the fear-strategy that terrorism, by necessity, brings.

For example, whilst it is a violent struggle to overthrow a government, it does not include a target-set. A freedom-fighter could overthrow a government through a prolonged insurgency that renders the government immobile, without targeting civilians in the process. Likewise, a terrorist may have the same objective to overthrow a government, but what distinguishes a freedom fighter and a terrorist is their methodology. Their goals maybe the same, but their means are different.

I can understand how you come to the conclusion that states can be terrorists. However, I think it's an important distinction. The abuse of power by states (that intentionally kill civilians, non-combatants) is a war crime. A war crime, to me, is more serious because typically states have more resources to commit these acts. Secondly, they have the geography to plan such attacks. Thirdly, it tarnishes the nation who are uninvolved in the decision-making process in committing these acts.

That's why I find it hard to sweep terrorism to state and non-state actors. There is a greater danger when a state partakes in these indiscriminate acts as opposed to a fringe minority.

It means that both sides were in the wrong, as far as we can tell.

How were both sides in the wrong!? Saville found unequivocally that the British Army were at fault, David Cameron went so far as to apologise. Please don't talk nonsense about such a sensitive topic when you clearly haven't got a clue.

That's my opinion – that the army were provoked by aggressive protestors. I don't care what David Cameron says he thinks. I'll express my opinion on any topic I wish, thanks.