There are much better ways to tackle a nihilistic existential crisis. EVA, Shoujo Shuumatsu Ryokou, Ergo Proxy all do it effortlessly, this, just doesn't make sense.
To get the objective assessment out of the way, the film scores well in technical aspects, cell-shaded 3D isn't that comical (bar's pretty low anyway), action sequences pack satisfying punches, sound and atmosphere befitting a major production.
But the frigging screenplay...... (spoilers)
The creator of Psycho-Pass focuses on one aspect of humanity in this trilogy - an insatiable desire to own, and control, be it oneself or the planet. The first film sets up like a shonen manga, hailing the main character's drive to retake Earth from Godzilla as an heroic battle against adversity. The second film reaches the pinnacle of the series by showing revenge in itself is an uncontrollable beast, and saving humanity by all means necessary would ironically destroy the very thing it proclaims to preserve.
So what's the message of the finale? A suicidal cult which the protagonist destroys, who then... chooses to commit suicide anyway???
So the protagonist cannot let go of his hatred against Godzilla, but neither is he willing to use nanometal at the risk of turning humans into destructive machines. What's his solution? Robbing other survivors' chance to choose by kamikazeing into Godzilla. Where went his shonen boiling blood to fight against adversity? His last act is the ultimate cop-out, against everything he stood for in the first two films, against what he just did a few minutes earlier! If he's going to choose death regardless, why not let Ghidorah get on with it? As to destroying the nanometal, its rediscovery is inevitable, this is the equivalent of leaving your homework for tomorrow - what's, the, point???
If the films wanted to leave on an ambiguous note, they could've left on a high note and stopped at film two. The third one is filled with nonsensical contradictions that turned the protagonist from a principled paragon into an indecisive joke.
As far as frustrating protagonists and endings go, this is Earthsea-level bad, yes, THAT BAD.