Concern troll pretends to support but uses "concern" to hurt morale (GD)

[Her Sister]

14. More: What is Concern Trolling?

Concern trolling is a form of Internet trolling in which someone enters a discussion with claims that he or she supports the view of the discussion, but has concerns. In fact, the concern troll is opposed to the view of the discussion, and he or she uses concern trolling to sow doubt and dissent in the community of commenters or posters. Although this practice originated on the Internet, it has since spread to the real world as well, with concern trolls popping up in a variety of places from network television to op-ed columns.

Artful concern trolling involves developing a believable persona as a supporter of a cause who has legitimate concerns. In an example of concern trolling, a group of people might be having a political discussion on a website about a candidate they support. The concern troll would log on and say “I'm concerned that this candidate might not be strong enough to beat the opposition,” or “I'm worried that the candidate's history in the legislature might be a problem in the election.”

Once a concern troll has sowed dissent or discord, often he or she can sit back and let the other commenters do the rest of the work. When a concern troll has done the job correctly, the discussion will split, factions will emerge, and support for the cause will have eroded. Concern trolling can also be highly distracting, as people band together to oppose the concern troll, rather than discussing serious issues, including valid concerns which should be addressed

http://www.wisegeek.com/what-is-concern-trolling.htm