"Only this poisoned spear pricking the heart of a sith can kill it quickly enough to save its prey. In this section of Kaol we are all armed with a long sith spear, whose point is smeared with the poison of the creature it is intended to kill: no other virus acts so quickly upon the beast as its own."
Different sith but perhaps the malignity of the Dark Side poisons a Sith Lord as surely as the Barsoomian sith contains the source of its own death