Actually 07 runescape account hate to correct yourself on something lore related, even so the “killing” of Ner’zhul was more of a metaphorical situation from the Crown of Domination, it really symbolized Arthas now being the dominant personality, not the only one.
This really is pretty much proven as during one of several quests for the Alliance within the Howling Fjord in places you view the Lich King he remarks that they “was a shaman”, Arthas certainly wasn’t ever a shaman making sure that means Ner’zhul is still inside.
To help expand prove this, on it same occasion inside the Crown, Arthas also “killed” his younger self Matthias Lehner, but it turned out later says Arthas must rip out his own beating heart only to get Matthias off his back, so he wasn’t killed from the Arthas novel either.
Another point out disprove this is the whole way Ner’zhul was crowned Lich King in the first place. He was condemned to reside in on and suffer eternally from the Crown of Domination by Kil’Jaden, who felt that runescape gold should suffer forever as punishment for betraying him way too long ago before Gul’dan took his place.