[YuuHikari]

Well yeah but why would they disappear and why did they chose to destroy humans than co exist with them? I don't have anything against the typical genocidal villain but I think its better to have villains that have logical or sympathetic motives.

Or we could divide the story's kaijin into those who want humans gone and those who are fine with sharing the earth with them and maybe have humans stuck between a war of both sides.