An ethical troll is anyone who claims or practices an ethical or political rationale for the behaviour which leads them to be called an Internet troll.
Once someone does claim such a rationale, their behaviour should be examined for internal consistency from an ethical, as opposed to behavioural, point of view. For instance, someone who strongly objects to Iraq policy might disrupt some conversation regarding its implementation - this is not evidence of a behavioural problem with the person doing so, but rather, an ethical problem with the policy that attracts such attention frequently.
Many such people cooperating can force a New Troll point of view to challenge existing systemic bias or groupthink (also called neutral point of view) but only if they are ethically consistent and politically discipined.
The freepers, for instance, consider themselves ethical trolls, but are less ethically consistent and probably not very politically disciplined, than they'd need to be, to take over forums like dkosopedia itself. Though they will no doubt try. Many wiki best practices advise respect for ethical trolling and troll-friendly policies in general so as to avoid over-reaction, disruption due to surface differences, and other administrator errors easy for enemies of the board/wiki's purpose to exploit. Over-reaction only enables the disruptors.