A basilisk, in this context, is a thought that kills you if you think it, which is excessive for this, and for Roko's. I mean a thought that breaks your cognitive processes in some way if you think it. Which I think is a fair way to describe someone who, on contact with the "you're murdering everyone you don't try to save" idea, is consumed with guilt that their every moment is not devoted with maximum effort to saving the world.