In the Bound Queen quest, Jullius Tallus is unkillable. The only way to get his key is to pickpocket him, but the quest description says to either steal or kill him to get his key, but the kill option is not possible.
Even using the console to remove essential from him does nothing, even using the kill command afterwards only causes him to kneel over in pain.
This has been happening for quite a few versions.
I've been having a look in TES5Edit, and he is classified as Unique, but that doesn't institute him as being unkillable.
Is there a fix for this and/or can this be fixed for the next version?