I think my main problem with the terrorist organisation is this quote (provided by Ignacio, but representative I believe of supporters of ETA)

On what basis should a legal government negotiate with gangsters/killers? In the Basque Country there is an autonomous government, with more powers than most in the Spanish constitution. I'm given to understand there has NEVER been a majority vote for the political viewpoints shared by ETA, either for the Basque government or for the central government. I understand that at most 20% have voted for extreme parties there.
ETA's position seem to be similar to a gang that say extorts money in a big city. Then the local police try to stop their activities. Various businesses are destroyed; a few people (not too many so as not to appear too bad) are killed. The gangsters then let it be known "negotiate with us on how much we are allowed to extort, and these attacks and killings can stop for a while"
I can see no government that could 'negotiate' under these terms...