Well he is certainly pitiable. Basically a kid who was seconds from death and latched on to person who saved him extremely hard, believing what Grace told him even more than Grace did. I think it is likely that the two of them bounced off of each other when inventing the ideology of the Apex, Grace with the belief that higher numbers were good, and Simon attempting to convince himself and others that the denizens aren't real people as a means to protect himself from the pain of being abandoned, in his estimation, by The Cat.
He had an interesting set up as a character, however the writers have him pull some turns that I find hard to believe unless he legitimately had some mental disorder, and not just Autism, but full blown sociopathy. Being unable to predict that Grace and Hazel would react poorly to his decision to kill Tuba. This doesn't really mesh with his previous characterization as a loyal follower who was quite devoted to Grace and the Apex. Sociopaths wouldn't latch onto other people in that way.
So yeah, interesting character set up, but I wanted the writers to do a better job with him. His fall from Grace (heh) was unconvincing.