Seraph Worldview

Though they're almost unequivocally viewed as the antagonists of the magical world, the Seraphs naturally don't view themselves that way. They have a different version of their history and outlook than most others would tell it, which can provide some insight into what motivates them to lead such a consistently harsh lifestyle.

The Immortal Flame

By anyone else's standards, the Immortal Flame is a demon ; a native denizen of the Otherlands, feeding on ambient and living magic and malicious towards most living things. However, while Seraphs share the same widely accepted views on the Spindle as anyone else, their beliefs question a certain distinction most others take for granted. What distinguishes a god from a demon? Both make their homes in the deepest reaches of the Otherlands. Both are overwhelmingly powerful. More often than not, both are far removed from the cares and concerns of human lives.
As the Seraphs see it the Immortal Flame is a true god, outnumbered and cast down by the gods of Genesis but still divine in nature. Their own mythology casts him as originally standing among the pantheon, but dissenting with their decision to loose the Reaper on the earth and being vilified and stripped of much of his former power as a result. They feel that his connection to the family and to humanity as a whole is one borne of pity and understanding, that the Flame saw humans outnumbered and overpowered by the other races in much the same way that he was. While those who've communed with the Flame more directly can often feel the sheer evil in that presence, most choose to justify it as the understandable result of an eternity to dwell on his oppression and betrayal.

Origins

According to recorded and generally accepted history, the Seraphs came about their demonic heritage in a fairly mundane way. In the earliest days of the Unified Era, thought to be sometime around the year 3500 BCE, a mage gravely overstepped their own limitations and lost control after summoning a powerful demon who would come to be known as the Immortal Flame. Killing the mortal who'd brought him to the material plane, the Flame was then freed into the world. He took refuge in human form to avoid detection and proceeded to leave a path of corruption and destruction in his wake, growing ever stronger the longer he went unchecked. Before being stopped and banished by the Reaper, the Flame fathered three sons on a human woman; two of these sons would go on to have children of their own, fathering the Naga and the Seraphs before being killed in turn.
According to the Seraphs, the earliest days of humanity saw humans pushed to the ends of the earth by the other races. They lived in the places nobody else saw fit to live in; the deserts, with no shelter from the baking sun and little sustenance to be taken from the land. When he was cast out from his place among the divine, the Flame came to walk the earth and saw humans suffering as he suffered, and took pity on them. He took human form to walk unknown among them, and gave them fire to light the darkness against their enemies and made the land green for them where it was barren before. And when he fell in love with a human woman, he gave her three sons to bless humanity with the bloodlines of a god.
As the Seraphs tell it, the gods of Genesis and their followers feared the power that divine blood could give to humanity, and sent their Reaper to turn humans against the Flame and his blessed sons, and it was at human hands that the his first two sons were killed. The Flame's grief overwhelmed him and brought cataclysm on the earth, creating monsters and demons; while humanity cowered, realizing their wrongs and begging his forgiveness, the the Reaper sought him out in his moment of weakness and madness and banished him far from the world once more. But a god cannot be killed, so the Flame reached out across the void even so to his surviving son Darius and directed him to protect the humans from what his madness had wrought.
And though Darius was his father's child and powerful beyond any mortal man, he knew that he would one day grow old and die. He walked among the humans who were humbled and repentant for what they had done, and he fell in love with a woman. And their union gave rise to the first Seraphs, who led humanity out of the barren places of the world.

The Reaper

Though their hatred of Reapers is certainly fueled by the role the Reaper played in banishing their god, Seraphs have a more fundamental disliking of the servant of the gods that goes well beyond old history. Despite being inherently religious themselves, they've always felt that the Reaper symbolizes a different, flawed relationship between mortals and the divine.
In their way of thinking, the Immortal Flame cares deeply for humanity and cares for their suffering; though they worship him as a god, they also consider him a patriarchal figure who wants to guide and protect the mortals he watches over. For better or for worse, the pantheon of Genesis and the Great Gods in particular have no such benevolence. They're inherent forces of existence, with a vast void separating them from the cares and woes of their worshippers. To the Seraphs, the Reaper is the ultimate symbol of divinity that cares nothing for mortals; killing indiscriminately in pursuit of a divine plan, with no thought for the suffering this killing causes.
Seraphs believe that only inherently evil beings could have such little regard for those who put faith in them, and that the gods of Genesis are worthy of nothing but active opposition. They hate the Reaper for the same reasons that most inhabitants of Genesis revere them: as the living symbol of the gods.

Guiding Philosophies

With these views of their own rule and origins taken into account, it becomes possible to break the principles that the Seraphs live by into a more simplified list.
HUMAN SUPREMACY
Despite holding a certain level of disdain for the ignorance of the human population, the Seraphs still consider themselves humanity's shepherds. They believe that humanity has an inherent right to exist freely and without fear, and hold a strong belief that the other races would see them destroyed if given the chance; ironically enough, this belief might not be entirely unfounded, as Genesis itself was created due to the Reaper Wynn Khajib holding the same fears. Seraphs, however, take this a step further. They believe that the only way to ensure the security of humans is through overwhelming dominance, and that thinning the ranks of the other races when given the chance is a necessary step towards this end.
OPPOSITION OF THE REAPER AND THE GODS OF GENESIS
While the Seraphs' philosophical and historical reasons for hating the Reaper have already been outlined above, it's also a matter of practicality. The Seraphs understand that the gods of Genesis don't directly involve themselves in the affairs of the mortal world and act only through proxy; as a result, the Reaper is the strongest single obstacle they can set against the Seraphs. Destroying the Reaper permanently would pave the way for the Flame's return, and has always been a top priority of the family.
RESTORATION OF THE IMMORTAL FLAME
The Seraphs' ultimate goal has always been to allow the Flame to return to the world, and they've strived tirelessly for centuries towards this end. Once restored to the material plane at his full strength, with the Reaper destroyed, they believe that no force could oppose him; he'd be free to bathe the earth in cleansing flame, destroying their enemies and unifying all of humanity under his rule. While there are certainly Seraphs who dream of rule by the Flame for more selfish reasons, many truly believe that the demon god would bring legitimate peace and a world where they'd no longer have to exist in the shadows as the world's thankless protectors.