Let us start with the definition of the Cycles. Each cycle must end with Ciro dying, and begins with Ciro being born again, with a new form. However, it is not a requirement for other people to be revived until after Ciro has been reborn.
Oric himself states that the cycle shouldn't be complicated so early; implying that he wants Ciro to continue onto Proxxy; even going as far as to tell us "Not to be a hero"; implying even further that we should not be kind or try to save people(Perhaps to get as powerful as possible)
The Warrens itself has been described as a Prison(will not provide quote, said countless times), with Oric being the Warden, and perhaps the Justiciar(Said by FFS). It is his job to see to the fates of his Siblings(Said by FFS), and keep everyone inside the prison. Due to being in Prison himself for so long, he sees that punishment is a reward to him, so he must punish the Others for their crimes.
So in his plans to keep everyone in check, and punished; he uses his Siblings to kill his Siblings for all eternity. Ciro being the optimal candidate for getting as strong as possible in the smallest amount of time; was chosen for the role of killing off his other Siblings. His Siblings' Punishment would be countless deaths for eternities, by the hand of another Sibling. It is even amusing to him.
However,the Warrens themselves are damaging to Soul Charge; and Oric, being the only one who hasn't yet died in the Warrens; requires Soul Charge constantly so that he doesn't become Illborn.
So in short,
Oric wants Ciro dead because it his eternal punishment in the "Prison" that is the Warrens, but also requires the needed Soul Charge because Oric hasn't died yet in the Warrens But while Ciro is alive, it is his duty to enact the Punishment to his other Siblings, by killing them.
It is the most efficient way of killing your own Siblings endlessly; because doing it yourself would be faaaar too draining on your mental stamina.