The survival of those movements (or at least the continued popularity of some of the ideology) has a good deal to do with recent Greek history and the current governance. I'm not qualified to try to explain European far-left politics to anyone but suffice to say that the politics of Western and Central Europe are a completely different game from the Balkans.

First, radicalism tends to be strongest when the principles underlying society have failed to produce results for common people, which seems to be consistently true with Greece

Second, whether it's "right" or "left" wing has a lot to do with the historical conditions. The Left is often strongest when people blame institutional figures (ie bankers, political leaders, religious hierarchy and powerful trans-statal forces like the IMF) and the Right when people blame subversive forces (ie ethnic minorities, counter cultures and progressives).

Why is it leftist more than rightist in Greece? Perhaps because Leftism has more credibility among the populace, and because the targets of Leftist ire tend to match fairly well with the targets of some of the overall Greek anger.

Why is it more violent in Greece? That I'm not sure about, but it does Leftist politics no service to kill people at random. I hope there are no successful attacks from groups like this; as a person with leftwing sympathies myself, I would hope that the goal of spreading the message can be met without widespread terror.

The coercive principle tends to mitigate against indiscriminate violence. Anarchists broadly believe in creating a society free from all forms of authority and coercion, which could be represented at its most heinous (say) by the taking of another person's life. And they're pretty consistent in rejecting terrorism. But then we have to define terrorism. They don't target civilians in theory, but they do make distinctions between workers and owners, "oppressors" and "oppressed", and an exception for police officers and other representatives of the nation-state. Kiesling mentions the much more extreme Sehta's "ostentatiously brutal contempt for life" which the other anarchist groups don't seem to have condemned. This includes the July murder of investigative journalist Sokratis Giolias.

There have been some interesting developments in anarchist strategic theory, too, that's worth a mention. For this I'd turn your attention toward "The Coming Insurrection" -- that little (but incredibly tedious) blue book which might be to today's left-wing radicals what "Quotations From Chairman Mao" was during the 1960s and 1970s. It encourages radicals to arm themselves, but cautions against armed struggle and "Iraq-style urban guerilla warfare, dragging on with no possibility of taking the offensive". Instead, it suggests radicals adopt an "armed presence" where the direct use of weapons is rare. And it argues that this is a form of pacifism (!) -- that by establishing an armed presence the radicals will develop strength, the nation-state will weaken, and pacifism will become "a sign of power, since it's only in an extreme position of strength that we are freed from the need to fire."

I suppose it's like the anarchist version of parking an aircraft carrier off the coast of some presumptuous little country. That is, the point is not to shoot anyone, it's to build up strength to the point where you don't have to. If I was in charge of Greece's national security thingummy, I'd interpret these bomb attacks as such a signal.