Didn't he slit Tiara's throat? He could have decapitated Riku to make it look like someone else's work.

[A nod.] He may know more than he's letting on, seeing how he's not saying anything in the first place. [He looks angry...but settled.] I think it's for that reason we should keep him alive a bit. Talk to him. See if we can find out anything else and incriminate him ourselves. A stray murder would be too suspicious.