I think both you and Eliezer are right. Eridu probably is sincere in his beliefs, but also has been reinforced by the attention he received into provoking people in a way that doesn't help his cause. He seems to even know this but still endorses his behavior:
I knew what I was going to get when I posted my first comment -- I just thought it'd be an amusing waste of my time, which has been roughly the case.
ETA: I'm not sure how we can try to help someone level up their rationality without at the same time giving them attention and risk turning them (by reinforcement) into a troll. The "hide downvoted threads" mechanism only limits the damage...
I just found out that a new website feature was implemented 2 days ago. If a comment is voted to -4 or below, it and all replies and downstream comments from it will be hidden from Recent Comments, and further replies in that subthread will incur 5 karma points penalty. The hiding, but not karma penalty, applies retroactively to comments in that subthread posted before the -4 vote.
This seems to be worth a discussion post since most people are probably still voting things to below -3 without knowing the new consequences of doing so.