From the Grimoire, we see that Sepiks is more than just a Fallen enemy.
Servitors have complex relationships with each other and with their Fallen crews. Servitors are attached to a Prime.
Sepiks Prime is literally connected to each and every Servitor. Every time we destroy a Servitor (like in this week's Nightfall), we are attacking Sepiks.
While the Kells are the political leaders of the Fallen, Servitors are said to be their gods - and the source of their life-sustaining ether. A Prime Servitor is a target worthy of even the greatest sacrifice.
Sepiks is not the aggressor, we are. We are destroying his Servitors and killing his people.
Worse, from my research, I have found a darker reason why the Servitors, and Sepiks Prime in particular, hate us. There's no Grimoire card for it, but there are clues hidden throughout the game that explain a horrible truth:
We kidnapped Sepiks Primes children and have spent possibly years torturing and killing them.
Servitors are said to be their gods
Think of the nature of the Fallen's attack. They are trying to take the Tower, the last city left to us. They follow the will of their Servitors and of the Prime, itself. Their actions reflect its will. They attack our city because Sepiks tells them to.
But why? Why not destroy us from orbit? Why not crash ketchs into the Tower until it's nothing, but ruins?
They can't. They can't because we have their children in the Tower.
Ask yourself what you might do if you were in their position, if it was your children being tortured and murdered.
It takes work to find them because they move, but if you know what you're doing, you can see them. You can watch Guardians experimenting on them, even killing them some days.
Truly, we are the monsters.