In a parallel version of our Earth, some decades ago a big attack was launched from an unspecified point, where insect-like kaijus (some as "small" as cars and houses, others as big as large buildings) swarmed a bunch of unsuspecting countries which were devastated in the process. As a result, a significant part of the continent (I still need to decide which one) was lost to these creatures before the attack could be contained by an alliance of the remaining countries.
Since then, technology has advanced significantly to the point where it allows for mechas to be created. The war with the insect-like creatures has never ceased, but it fluctuates between periods of relative calm when the number of creatures recede and show a less violent attitude, and short periods of desperate war when humanity does its best to contain the new swarms of monsters that cross the front. The remaining countries do their best to rebuild, recover some territory and advance their technology during the years of calm, and some can even maintain some level of prosperity and welfare state. At this point, the society has got relatively accustomed to the cycles of war and rebuild, and there is no sign that the war will be won in the near future regardless of what the state propaganda says.
One megacorporation has profited significantly from the situation. They manufacture a wide range of commodities used by the public and is usually perceived very favourably by the masses. That is the same corporation that is now developing most of the mechas that collaborate with the military during the periods of war. For the record, I am not sticking to 100% realism here since these mechas are somehow inspired by the Evangelion ones, with a combination of mechanical and technological wonders but also "something else" that allows for some physics constraints to be surpassed and for some synchronization with the pilot that allows them to feel the mecha as an "extension" of his body. The corporation is using this "power that they do not really understand" for this and other practical applications, but also trying to study it in depth to fully comprehend what they are dealing with. I have yet to decide the specific nature of this power, which to give some examples could be something as vague as "the essence of God" or "the source of life and evolution" or as specific as a "weird material that composes most of the inner core of the Earth and subtly alters theta waves", and anything in between. This supernatural component is probably linked somehow with the insect creatures that keep attacking humanity.
The plot twist is that the megacorporation is indirectly responsible from the original appearance of the kaijus in the first place, and also DIRECTLY responsible from the periodic attacks that occur from time to time. I mean "directly" as in "they take some actions despite knowing that they will lead to new attacks". Of course, both the public, states and even most of the corporation employees are oblivious to this fact, and only a select group of directives and high level employees in the megacorporation know the truth. I do not want them to be cliched villains, but to have a real motive for these decisions. They do not want to end humanity, but to achieve some goals that might even be altruistic (or at least not completely selfish) and for the "greater good", even if that means that millions of people will suffer and die due to this horrific war as a consequence. The attacks might be the desired outcome from their actions or a side effect from them.
What could be the motives that lead these elites propagate the vicious attacks?
Edit: Due to some confusion (I might have worded it out poorly) I must stress out that these guys are not EVIL (with capital letters). They might be real assholes and like some degree of hedonism, but their goals is not just to profit for themselves. They do have some kind of "higher purpose", and they are willing to sacrifice a lot (as these attacks prove) to achieve it. Their rationale might be flawed or just plain wrong, but they sincerely believe it is the best course for humanity. These motives should be something that you could emphasize with even if you disagree with them or they horrify you, like you can probably understand why something that belongs to a sect and truly believes that the world is going to end UNLESS is capable of some truly horrific acts in the name of some higher purpose (just an example, these elites are not necessarily brainwashed).