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.