I don't think you need the victim complex to be so heavy, at least not until after he ascends. I suggest a more gradual descent into evil representing his faith in the world being shaken (parents lied), damaged (society is unfair), then destroyed when he realises the world is fake.
Otherwise, kos sounds like he is a bit messed up as well and I can't really understand his motivations to make an unstable, evil wizard a god other than to create an antagonist to fight.
EDIT: And what about the replacement god of false information?
Sounds more like you are playing Primal Order than Pathfinder. When you say BBEG, it usually means an antagonist that the players can be challenged with, yet can still be defeated.
Throwing around the whole deity thing makes me think you are running Primal Order (where everyone plays gods), or a high level mythic campaign.
The BBEG sounds like the ultimate "careful for what you wish for" person that is now throwing a tantrum. He will be difficult at best to negotiate with as he isn't interested in "talking to pawns and figments of other's imaginations". He doesn't seem interested in deeper philosophies of WHY creation is simply a game (what forces made the game the way it is?). This does limit interaction with the BBEG to simply "I apply my weapon to his face".
Also, it is better to know what personalities you have opposing the BBEG. I would save this type of BBEG for a group that has at least one philosophically minded PC and one jaded gambler type PC. Otherwise the backstory would be lost as the PC group would just be interested in solving the problem with violence, not really caring what the backstory is.
Your BBEG sounds somewhat deranged. He thinks/knows the entire reality in which he exists is a figment. He is not going to listen or reason with the party. He is also powerful enough to shape reality around him through his magical ability - further convincing him that it is not real.
There are a few parallels with an evil version of Neo from the Matrix here. In fact the whole plot could draw some inspiration from the Matrix, but without ever experiencing the world outside. Actually, just thinking, that might be a good starting point. Reframe the entire Matrix storyline with the Architect and the machines as the good guys and Morpheus and Neo as the bad guys. Kos is essentially Agent Smith, who mistakenly picked Neo as his successor and has now engaged the party to stop Neo.
The really hard part is trying to work out the BBEG's plot in detail and how the party can stop them. Are you considering playing a high level/mythic campaign from the beginning or building up to it from a low level?
Building up from low, the party is level 6 right now and i plan to go on for 2 years or so. The setting will ensure a wide variety of powerful pseudo-demons to kill for XP for continuous levels
and must i say, your idea of the matrix is utter, flawless, gold because everyone in my group has seen and loves those movies.
thank you so much Hugo Rune
Happy to help
Are you considering dropping clues as to the fact that the reality is not real? If so, that might give you an overarching storyline to fill in. Each clue becomes the subject of an adventure. You may want to start by developing the storyline and plot points for the BBEG to achieve his goal without consideration for the party. The write the players' storyline. Then munge the whole together, tweak and edit and rework until it makes sense. You should then have a campaign framework and matrix of adventures. Each adventure develops the storylines and can show the BBEG's goals being met or the players - or sometimes both.