This suggests that remaining Soul Charge is transferred from the being that got killed to whatever killed it. It also implies that there is one soul that charges much faster than others. Given how quickly Ciro's charge filled up when Al died, I think it's safe to say that Ciro's fills the fastest.
So now Oric has a motive for killing Ciro: He wants Ciro's Soul Charge. Well, why not kill the other Siblings? Three reasons:
1. The other siblings don't have access to the Oversoul. Only by killing Ciro can Oric harness its power.
2. If Ciro kills the other siblings, his soul will be stronger when Oric kills him/her (gender pronouns are hard with Ciro).
3. Oric doesn't approve of slaughtering the Six Sinister Siblings, or of them slaughtering each other.
The Thief: You hate this person. He is the one who originally took your Soul Star and imprisoned you here for defying him. You actually aren't sure what you did to anger him so much, but maybe it had to do with killing people? Brutal, selfish murder usually makes people mad.
So, plain and simple,
Oric wants to kill Ciro so he can harness the power of the Oversoul.Interesting implication: Did Ciro absorb Yaos's soul when she defeated her soul?
Alternative theory:
Oric wants to live forever:
YOU are not the thief. The other one is the thief. You remember now. You cannot die here because the thief is going to live forever.
You cannot let the thief live forever.
There are a select few who can access the Oversoul. I believe we've seen 3: Ciro, Oric, and Cain. The Oversoul grants extreme power; possibly even enough to kill someone else who can access the Oversoul. Oric imprisoned Cain and stole his Soul Star so that Cain wouldn't be able to use the Oversoul to kill him. Now Ciro is the only one Oric has left to worry about.
Summary:
Oric wants Ciro dead because Ciro is a threat to the eternal life that Oric desires.