I lean more toward an "Other" response, because over time the reasons will change and will also be manipulated by those in the higher command ranks (as much as those may or may not matter for what develops on the ground).

I for one believe that over time the basic ideology of a terrorist group (in the classic sense) really doesn't matter. They become addicted to the cycle of revenge killing, or it becomes so institutionalized in their operations that the original reason(s) for the killing don't matter. They may always have an IO reason for their killing, but at the ground level that reason is more a slogan than an actual belief system.

That said, it is always important to make some fine distinctions with these groups. Some, especially the political wings, are obviously open to maneuver. Others, such as the hard-core jihadist/Provo IRA/whatever cells, are not.

For the upper ranks, I would say that motivations tend more toward a mix of response 1 and 2, with the shading depending on the group in question. There is always a cultural component, but that can be triggered by policy decisions as well.