Hattrem are incredibly calm pokemon who spend their lives in solitude, at least in the wild when they have a choice in the matter. These pokemon not only read emotions, but also feel them, for in this species the distinction between the self and others is notoriously incomplete. And to their uncomfortably developing brains, strong emotions are experienced solely as pain.
When their own actions as monarchs did nothing but discredit them, and the power and mystique of Zacian and Zamazenta were no longer enough to keep their subjects in line, Galar's last kings resorted to Hattrem to maintain order. Hattrem are not the strongest pokemon, but they are more than capable of beating up an unarmed man (or, for that matter, woman or child) and did so repeatedly whenever they encountered strong emotions in public; should their foe be able to overpower them, they could always call for police backup.
The diarchs justified Hattrem's efforts as necessary for stamping out crime and sedition, but rather than preventing people from feeling anger and frustration, they stopped them from feeling anything at all. Unprovoked attacks on sporting events, weddings, and even funerals were commonplace, and rather than calming the people of Galar, their anger only grew with every abuse.
Until that generation of Hattrem realized their violence was not creating a calmer world. The flight of the Hattrem to Glimwood Tangle – all at once, although surely planned and debated well in advance via telepathy, in conversations human historians dearly wish they could read – is usually held to mark the start of the Galarian Revolution. But the Hattrem responsible are Hatterene now, if not dead of old age, and instinct is not easily overcome; contemporary Hattrem do not patrol Galar's cities, but they are every bit as violent as their repressive predecessors.