The Platinum dex entry said that Giratina was banished to the Distortion/Torn World, because of being violent. However, Giratina doesn't do anything brutal in neither Platinum Version, nor Sky Warrior.
In Platinum, it attacked Cyrus, by shaping its wings like claws. Apparently, Cyrus' actions were causing turbulence to the Distortion World, thus infuriating Giratina. Similarly, the Dialga v.s. Palkia battle in the anime, filled the reversed universe with some kind of purple smoke, which is why Giratina assaulted Dialga.
And no, Giratina didn't try to eat Shaymin. In fact, it never really paid attention to Shaymin in the whole film. (How could it pay attention to something 100 times smaller than it, anyway?)
For all these reasons, I think that Giratina is pretty much like the huge Tentacruel in Porta Vista, quiet in general, but really violent, if gone berserg.