There is a third option, the one used in the book Ten Little Indians (though I read the original version that had another title). Spoilered for those who want to read the book (which I really do recommend as a good mystery)
SpoilerIn Ten Little ******* (the original book), the murderer is actually one of the first victims... he fakes his death and then finishes off everyone else, before killing himself. The perfect crime...
Edit: hey, who'd have thought it would be filtered out. Shows why they changed the title I suppose.