ok here's a random thought, but it makes perfect sense to me.
ok apparently the 1st horocrux was Tom's diary right? Well to split his soul, he'd have to kill someone. I don't know/remember what grade he was in when the Chamber of Secrets was opened or when he was told about horocruxes, but i think it makes perfect sense that he opened the CoS so that he could kill someone, even if it was indirectly. Moaning Myrtle was the "key" to his 1st horocrux! (well, from how i see it..:S)