It wasn't emotional attachment to crota that brought oryx, you are using our human concept of love. Hive religion is extreme survival of the fittest, the best killer wins. Not only do they seek to survive, they actively seek out to eliminate the weak, and they actively seek out the strong to test their own strength against (why they spend 20,000 yrs killing each other, to become better killers). Oryx tossed crota to the vex and could have cared less if he died. He only "loved" crota once crota became an excellent killer. So when we killed crota, we elevated ourselves up the chain of command of killers. We were better killers than crota (because we defeated crota), and thus we were sought out as challengers. Last few verses of the book of sorrows. Oryx sought us out because we were good killers. Either oryx would become a better killer by defeating us, or oryx would lose and we would rightfully replace oryx as the superior killers. Which in a weird way, satisfies oryx desires. Whether him or someone else, he wants the best killer to win.