So David's motivation is wiping humanity out because he agrees with the gods (the Engineers) that humanity has grown cruel, weak and full of greed; leaving him effectively trying to follow in the footsteps of the LV-223 Engineers, setting about completing the mission they started, by perfecting the bioweapon they made?
It seems clear now from some of the descriptions appearing online of the bombing scene, that the victims on Paradise are not the Engineers, but a more primitive humanoid race, that the Engineers have created in the same way they created humanity on earth. And just like ancient man, this new humanoid race worships the Engineer's as gods; hence all the giant statues in the citadel plaza. The citadel is their Mecca where they gather to pray. This is why they look on in awe when David's ship arrives, because they believe it's their gods (the Engineers) returning from the stars?