Heres my two cents:
I think that Giratina represents death, hence that it appears in an ancient cemetery. Giratina seems like it is based on a basilisk, which causes death by a glare.
Now, for the origin, remember, this is just a theory:
Since Palkia and Dialga are Arceus' halves, what is Giratina? Giratina may have been created to balance out thw two. Dialga and Palkia represent Time and Space, which is pretty much the fabric of reality. arceus represents the world, in my opinion. So, with Dialga and Palkia usually fighting, Giratina must be used to put an end to that fighting, much like Rayquaza. Death usually represents the end of things.
Well, that's my theory =P.
~Gira
I chose to be a girl no diamond because I can.