Death (I get it's the destruction pokemon, but let's face it, it's the life/Death duo) =/= Hell
They may be thematically linked but they are two very seperate concepts.
Yveltal isn't neccesarily evil. Death, as represented in the Arcana, often signifies a new beginning. Paving the wway for the future. Climb a mountain of corpses to see a beautiful new sight.
Hell on the other hand signifies punishment. Presided over in religion by the first to be punished ( I think the fallen angels come before Adam and eve at least), the first ANGELS at least to be punished. Giratine is the poke-equivalent of a fallen angel, punished to a seperate realm. That said this is why giratina would be more relted to the Devil, what with the possible links to biblical myth.