Alien intelligence is not required to follow human reasoning.

The Lords of Alpha Centauri could run a long-term social engineering program on Earth because they believe capitalism, conflict and social darwinism are objectively Good for You and we need to be purged of the folly of humanistic ideology before we can be allowed to join the galactic civilization market.

What I can tell you is that no rational spacefaring civilization would need to resort to social engineering if they just want to kill us. Just toss a bit (or a lot) of spare delta v on a sufficiently large asteroid (or five) and humanity goes the way of the dinosaur.