I understand the idea of public image, and I do see where you’re coming at - there are always going to be people who take the actions of the fringe and identify it as the actions of the mainstream of that person’s affiliations. Just as there are people who take the actions of this guy and use them to reach assumptions about blacks in general, or the antics of this guy and draw conclusions about all gays, there are people who will look at the actions of the guy you linked to, and draw conclusions about all animal rights organizations. We know this is true, because you’ve done it in your OP.
What, I think, is puzzling most of us is what you expect anyone (other than law enforcement) to do about this. If his words are somehow tainting Greenpeace, what is Greenpeace supposed to do about that? They’ve already publicly disavowed violent action, and taken steps to purge such people from their ranks. What else can they do? Give up on environmental activism? Because some random nutjob on the outer edges of their political axis said some stupid, reprehensible bullshit? That seems a tad unreasonable to me, but I’m generally not sure what else you’re looking for, here.