I have thought about that, tumblindice, and I can only give you my biased answer.
They want to destroy what they created because of what we have become.
I ask myself if I created mankind, what would I do with it?
The answer I came up with, considering our world today, is to start again.
Which refers me to a line in the movie,”Sometimes, in order to create, you must first destroy”, which ironically is a Joseph Stalin quote, which is kinda creepy...
Sounds like that creepy vampire in `Thirty Days of Night’: “That which can be broken, must be broken.”
What David the robot said seemed like a clue: “Why did you make me?”
S.H.I.E.L.D.’s Nick Fury is right, ayup ayup. We need bigger WMD weapons so we can tell whomever drops in to get the `ell off our lawn!