I'm pretty sure it was him being murdered, here's how I drew that conclusion:
1. It was just told that Ai has tendencies to murder and enjoys taking pleasure from it; her mother said she'll use her own interests to control others. Which leads us to her manipulation of Aoi to kill him and his...
I love this distorted ending, Aoi lied in his journal entry about him committing suicide, just to protect the very person who would obviously betray and kill him. This was a good read for its length, not too short and not too drawn out.
I'm guessing that the author wants to move onto other projects and/or having to write out even more story for these characters probably would've ended up worse instead of just doing a timeskip.