The Nature of the Seeds

So the Seeds are Investivores — think of Chiri-Chiri — in that they feed off investiture. How exactly they feed off investiture depends on the world, its environment, and its shards. On Scadrial, for example, they do not need to eat souls- they can simply bask in the radiant investiture of the core, no need for anything more. That's not to say they wouldn't eat a soul if it walked into their mouth, it's just that it's not worth the effort, nor is it very healthy for them, as the identity corruption can cause indigestion, and the angry shards can make their lives difficult. They're not all evil, either, and some wouldn't eat a soul if it walked into their mouth. They'd view such a thing as wrong and would reject it. Depends on their upbringing, really.

Their overall intent, however, is not to consume. It's to create, specifically, great pieces of art. How they do this, what powers they possess, all depend on the world and shards they landed with.  

When a seed lands on a world, they align themselves with one of the resident shards, feeding on their investiture over the other, and aligning more with their intent. If there are two, the pair splits and one goes with each shard. One goes with Honour, one with Autonomy. Both Seeds of the Pair are not identical- one is active (magenta), while the other is passive (cyan). If there are three.... Well. You'll see.

In the act of landing on a world, they Connect themselves heavily to the world itself, inserting themselves into magic systems, redirecting ambient investiture to feed and grow. If they're given a blank slate, they'll transform the world into something visually striking. What? No clue. It'll integrate whatever investiture there is around. If there's already an ecosystem, they'll integrate, rather than corrupting it into something new. They may add on top of the magic system, adding their own spin, but they will not break it.

After they've integrated, killing them normally would do bad things to the world. Think BAM. They've connected themselves to the world, and removing them abruptly leaves a wound in its very soul. To use the same example of Scadrial , killing them after they've integrated would perhaps disrupt the growth of their crystals, and they would gradually grow more and more impotent as their world bleeds. 

TLDR: Don't kill them once they've landed. Kill the innocent, unborn babies before they grow up. You can also do other things, but hey, know Alph knows you pretty well, and expects you all to be evil.
 

I can no longer feel