Concern Troll is concerned:
In an argument (usually a political debate), a concern troll is someone who is on one side of the discussion, but pretends to be a supporter of the other side with "concerns". The idea behind this is that your opponents will take your arguments more seriously if they think you're an ally.

A concern troll will pretend to care (about the opponent's health, well being, political activism, lifestyle, choices, etc.) while in reality, he or she only wants to cause self doubt and fear. A concern troll tries to undermine that which he or she pretends to support.

What Is Concern Trolling? Watch Out For This Subtle Form Of Shaming
Urban Dictionary defines a concern troll as "someone who is on one side of the discussion, but pretends to be a supporter of the other side with 'concerns.'" In other words, it's someone who pretends to support you but couches their disagreements in the form of "concerns," which allows them to justify criticism as the result of worrying about you. "I'm on your side," they say, "but you shouldn't do X, Y, and Z. It looks bad to some people — not that I agree, but I thought you should know."

Wikipedia:

A concern troll is a false flag pseudonym created by a user whose actual point of view is opposed to the one that the user's sockpuppet claims to hold. The concern troll posts in web forums devoted to its declared point of view and attempts to sway the group's actions or opinions while claiming to share their goals, but with professed "concerns". The goal is to sow fear, uncertainty and doubt within the group.

concern troll
A person who posts on a blog thread, in the guise of "concern," to disrupt dialogue or undermine morale by pointing out that posters and/or the site may be getting themselves in trouble, usually with an authority or power. They point out problems that don't really exist. The intent is to derail, stifle, control, the dialogue. It is viewed as insincere and condescending.

Enter the Concern Troll

But I think it touches on a larger trend in opinionating.

Enter The Concern Troll.

There are two ways of disagreeing with someone. One is to disagree openly. The other is to say, “I support you, but you’re doing it wrong.”

The second is insidious.

The second is condescending, insincere, manipulative. It even says so in the Urban Dictionary definition.

The darkest moment is always just after the concern trolls start pouring in.

“I’m with you,” the concern troll says. “But surely you must see how this looks to people. Not me, of course. But other people. They might think horrible things of you. People might think you were self-centered, fat, slow, rude. Not me, of course. I’m with you. I have your best interests at heart. That’s why I want to warn you. I, you see, know how this ought to be done.”

Dems: Ignore 'concern trolls By Markos Moulitsas - 01/09/08 11:40 AM EST

In the blog world we call this “concern trolling” — offering a poisoned apple in the form of advice to political opponents that, if taken, would harm the recipient.

A Concern troll:
A concern troll is a person who participates in a debate posing as an actual or potential ally who simply has some concerns they need answered before they will ally themselves with a cause. In reality they are a critic. Concern trolling in geek feminism communities can result in continual reversion to Feminism 101 discussions in attempts to appease the troll's concerns, frustrating attempts at more serious discussion. Concern trolls are not always self-aware; they may also view themselves as potential allies who have just, oddly, never met a feminist opinion they liked.

Concern trolls can be identified primarily because they will retreat from, rather than engage with or be convinced by, answers to the questions they pose. They may repeatedly ask a certain question in feminist discussions without ever absorbing or replying to answers from previous discussions. They will often back into typical anti-feminist arguments, such as expressing concern that an argument is too "extreme" or a feminist too "strident" or even "hysterical". Another common tactic is insisting that some subjects are more important than others, for example, that media depictions of women shouldn't be criticised while violence against women continues.