Simple, Most Likely Plot (spoiler)

I thought at first that this planet they go to is [u]not[/u] the Engineer's homeworld because it is completely barren, then I read that it [u]is[/u] actually their home, so combine that with the photo of the long dead engineer in the tunnel and it tells me that they had an extinction level, biotechnological, accident and wiped themselves out. If they had also seeded Earth a long time ago, and sort of forgot about it, and you have us show up telling them that it is fully habitable -- the simplest conclusion is that the engineers think of it as their world for the taking since they created it in the first place. That is why we can't let them escape and why we kill the last of them.