Jj the chihuahua wrote: You may have to do really cruel things in your previous life but refer back to the anime of how Ciel Phantomhive became a demon.
That part of the anime isn't cannon. Yana hasn't said anything on how Demons are born (If they're born at all). Sebastian said on how/why he was summoned the first time he met 'Ciel' but that's about it.
Phantomhive servant mine wrote: To become a grim reaper you have to commit suicide but how do you become a demon?
You seem to imply there was a time he wasn't a demon.
If we go by Christianity (the type of demon he is from), he was an angel who rebelled from God along with Satan who felt they were greater than God. He was cast from heaven and thus became a demon.
