Sekto is an Octigi.
Obviously, they latch onto a victims head, and take control of it's body. Sekto, as seen in game, latched onto the Olden Steef's head. I imagine that the Olden Steef was sort of the Steef leader, seeing as how he had the biggest horns (you can see the Olden Steef's horns mounted in the phone call video).
He dammed the river to build up water for Sekto Springs (bottled water company). He killed off all the Steef, at first, because they would never allow him to build a dam, because the Grubbs need water to survive, and Steef are Grubb guardians, so they kinda go hand in hand.
But I think after the first few Steef slaughters, and Sekto was able to build the Dam, hunting Steef became sort of a sport for Sekto.
That should answer everything.