Today, Andrew Sullivan responded to a dig at his change of tune on treatment of prisoners. It actually hasn't been the most serious dig he's taken for chilling to the Bush administration, but it made me think, not for the first time, "Why do humans feel a need to punish apostates, religious, political, and otherwise?" More hated than the person who's always had an opposing position is a former ally who's taken it. In politics, it can be explained as another facet of treating debate as a sports match, where winning as opposed to understanding is the goal; in such a climate, someone who makes a rational change of mind is as crazy as a soccer players who's begun intentionally scoring own goals. Atheists even go after apostates sometimes, as in the response to prominent-atheist-turned-deist Anthony Flew's endorsement of Intelligent Design. Maybe the responses were needed to remind people of the flaws of some old arguments, but as I read the one above, I couldn't shake the feeling that the reason anyone went to the trouble was the urge to punish apostates.
As for my question of why, all I can think of is it's some kind of tribal instinct, though that doesn't satisfy my wonder at such spectacles.