You've no doubt heard about the strange and tragic situation in which James Jay Lee strapped bombs to himself (or what was thought to have been bombs) and took a gun into the Discovery Channel building. He took hostages and demanded that the network executives create a new program which would emphasize Malthusian themes, themes which would convince people to stop having "disgusting babies" and instead save animals.
He was eventually killed by police.
Wesley J. Smith wisely resists the temptation to blame Lee's insanity on the extremists in the environmental movement (though their goals are basically what Lee demanded - 1, 2) but instead urges fairness and genuine accountability, showing the difference between an individual's unhinged mind and the organized violence perpetrated by PETA, ALF and other eco-terrorists.
But one nut does not define a movement and people who argue in the public square on behalf of controversial issues are not responsible for the actions taken by said nuts. Thus, Al Gore has zero responsibility for this terrorism, even though his movie is propaganda, just as technology skeptics have no responsibility for the Unabomber, or the pro life movement for the man who murdered the late term abortionist, Dr. Tiller.
However, when a group of ideologues organize for the specific purpose of engaging in terrorism/lawlessness as a tactic–such as the Animal Liberation Front (ALF) and the Earth Liberation Front (ELF)–that is a different matter altogether. In such cases, it is incumbent upon the movement leaders, in support of which the terrorism is being mounted, to unequivocally denounce such tactics and cooperate with authorities in bringing the responsible to justice. Silence or winking approval in such cases amounts to praise by faint or non-damnation. In that case, I believe some blame can fairly be laid at the movement’s doorstep.