I think "disillusion" is misused here. It's not his disillusion that was shattered, but his illusion, leaving him disillusioned. We are disillusioned when we find that something we have (strongly) believed to be the truth is in fact not the truth, or at least may not be the truth. It's usually a very troubling experience emotionally. "Clutching at straws" is a metaphor for hanging on to something flimsy or weak for dear life, out of desperation, panic, or tenacity. It refers to straw as a material ("saman"), which is considered weak, not to drinking straws. Imagine a drowning man hanging on to some straw to avoid drowning. In this case, the man coped with the disillusionment he was experiencing — which “remov[ed] the linch-pin that was keeping his whole world in place” — by telling himself that he was bad, a flimsy thing to hold on to.