I have a few ideas:
1) It's a "basilisk", i.e. an imaginary lovecraftian threat that doesn't even make sense outside of some highly particular and probably wrong belief system. (That's not my definition of basilisk, but it is what I think of such claims.)
2) Some mundane fact about the difficulty or danger of actually trying to save the world (in the specific sense of shaping a singularity) has made his blood run cold. It could be the existence in the real world of powerful evil cliques; it could be the psychological potential of joining them, or just of becoming selfish in an ordinary sense.
3) I remember when I was 22 and realized (according to the plans I had at the time) that it might take me eight years to save the world. That was very daunting, because at the time it looked like it would be a joyless, stressful, solitary existence, for an unimaginably long period of time. And as it turned out, I didn't even get it done... Will could be fleeing the responsibilities of his "position" - I mean his existential position, which surely includes the perception that he has the potential to make a difference in a huge way.
ETA 4) He wants to create a barrier (a "credibility barrier") between himself and his former associates in SI, so as to develop his own thinking, because there's a systematic deficiency in their outlook and he must avoid the temptation of working within that paradigm.
Thank you for your cooperation and understanding. Don't worry, there won't be future posts like this, so you don't have to delete my LessWrong account, and anyway I could make another, and another.
But since you've dared to read this far:
Credibility. Should you maximize it, or minimize it? Have I made an error?
Discuss.
Don't be shallow, don't just consider the obvious points. Consider that I've thought about this for many, many hours, and that you don't have any privileged information. Whence our disagreement, if one exists?