Your theory is nonsensical.
1) all things come to an end. The fact that's the case doesn't imply some kind of evil "devil".
2) No, Giratina is not widely accepted to be a "devil". In fact, it's a part of the creation trio that Arceus oversees, along with Dialga and Palkia. Overseeing Giratina directly implies that it's not Arceus' equal and opposite. While it was supposedly banished to the reverse world for being violent, nothing is ever implied that it want's to destroy or end the world (universe). In fact, it's been shown in the anime to be very gentle when it wants to be.
3) We've seen the reverse world. It is NOT the "afterlife". It's a different dimension that mirrors the Pokemon world. Not to mention the fact when we saw the reverse world in the anime - Giratina was alone there. No ghosts, no spirits, no nothing.
So no, Giratina could not unofficially referred to as the Omega. There is absolutely no basis for your theory. Theories are supposed be developed based on facts and evidence. Not just making things up as you go with no evidence supporting it.