It certainly isn't the first time ETA have resorted to violence. But to put it in perspective, ETA have been carrying out terrorist actions for thirty years or so. In that time the estimated number of murders they have committed runs to about 800. If this is them they have increased their death toll by nearly a quarter in one day.
I really like the Basque people, spent a while in SW France, N Spain during one of my university summers. Anthropologically they are interesting, quite different to the Spanish and French 'normal' population. probably most closely related to the Welsh. Believed to both be remanants of an earlier migration to Europe than most of the present inhabitants and were driven to the corners of Europe by them. The Basque language is totally unrelated to any other language on earth as far as linguists can tell, closest in fact are some Inuit languages. The Welsh are believed to have shared a similar language orignally but lost it in favour of a later Celtic tongue. Basque words have lots of 'x's in them.
A lot of Basque resentment is due to Franco. Guernika was/is a Basque town and the subject of the first civilian targeted air raid. Under Franco all regional languages and customs were banned, Spain has always been a very diverse country. Catalan's were severely affected by this too. All this is no excuse for terrorism however, nor should we really call them separatists, especially if this mass act of murder is them.
Strange thought, but Spain has been a battleground before and for many long centuries. However you want to paint it, much blood was shed taking Spain back from the Moors who themselves were reasonably tolerant rulers. Responsible for some of the most beautiful architecture in Europe too, went to Granada myself just after New Year.
I'm leaning towards this not being ETA, or it being a new group equivalent to the 'Real IRA'.