Himizu imo is mostly about nihilism, trauma, and persecutory delusion.
Sumida's distorted outlook on the world perceives an overflow of disastrous consequences if everyone's dreams were to be realized in this small world, that it's okay to not be special, that he shouldn't change the status quo himself and live as if he had never been born; a 'temporary' life.
His familiar trauma through his mother's abandonment and mental illness gives birth to a relentless, stalking monster that grasps at his own mortality, eventually leading him into patricide, vigilantism, self-destruction.

Within this small, monotonous, and confining world, those around him adopt a different mentality. They possess aspirations and the determination to pursue them. Among them is Keiko, a girl in his class, who ardently attempts to shake Sumida from his perilous mindset. Gradually, a transformation ensues as he begins to find purpose in his life - the aspiration to do what is right, for once. Yet, the definition of "right" becomes a complex question. Who holds the moral high ground? Who gets to dictate such judgments?
Sumida's belief in purging the "bad guys" around him stems from his conviction that it will liberate him from his sins and trauma. Obsessed with achieving freedom and reclaiming the image of a virtuous individual, he embarks on extreme measures, driven by a lifetime of self-perceived insignificance.

What I love most about this film is that it ends on a hopeful note rather than its manga counterpart.
However, I will say that despite Sono's captivating directing & the immersive ost, the manga by Minoru Furuya captures this story's themes way better.