I wonder if there was a "God" to be killed in Zancrow's case. He's one of the ones that Hades gathered up specifically to be taught Lost Magic (Fire God Slayer magic in his case). I find it odd that Hades would just have a Fire "God" around to teach anyone he brings in. And it would seem like a waste for Hades to have said Fire "God" killed off when it's likely he could have made a Fire God Slayer army. I'm sure there's no limit to how many of the same type of God Slayers there can be, after all.
Chelia's case is a complete mystery. If there was a Sky "God" to slay, I doubt she would have done so. Unless she's secretly batshit insane, that is. I suppose the process by which one learns God slayer magic will be delved into someday.
Anyway, A God/Dragon doesn't necessarily have to be slain in order to take up the mantle of God/Dragon slayer. Look at Sting and Rogue. They claim to have killed their dragons -> No one believes them -> People still consider them Dragon slayers. To be a literal (or true) God/Dragon slayer, as Rogue and Sting claim to be in their cases, you probably should kill them. But it's the magic they teach you (God/Dragon slayer magic) that brands one a God/Dragon slayer.