Did I miss the scene in Independence Day where Will Smith sat down and asked the invading aliens about their motivations? Is that some DVD extra? I don't think the Shivans are interested in territory (they don't seem to conquer, only secure nodes, preventing transit), resources (again, they leave resources behind) and we don't know anything about them socially to even consider ritual. In short, I dont' see your link between the characterization of Shivans and the aliens in ID4. My take on Shivan motivation has a very simple parallel in modern medicine. When you cut out a tumor, you take some healthy tissue with it to insure that no unhealthy cells remain.
Er, yeah actually. There's a scene where the captured alien tries to use telepathy on the President,blah blah - revealing them to be a sort of race of intergalactic locusts....
Anyway, my theory on Shivans is pretty simple. They're evil. Simple, instinctual animals with a killing instinct, yet also intelligent enough to form plans and identify clear targets. i think that's their purpose - to kill anything in their way.
Of course, that doesn;t mean they are the only things out there. And it doesn't mean that they can't be scared, or that they can't run away.