Evan hadn't killed the Sixth Finger of the Demonic Hand, Seith.
After incapacitating him and weakening him with debuffs that limited his power, he handed him over to Yetu so that Yetu could have his fun and as many rematches with Seith as he wanted.
Even then, Evan had given specific instructions not to kill him.
He had also used the Slave Brand to command Seith against taking his own life, which would cause his soul to return to the First Finger.
Even if his soul was destroyed, the death of any 'Node' of the World Formation would alert the First Finger, and if Seith were to die at this juncture, then the First Finger would instantly order Deram, the Fifth Finger, who was currently in the Wild Lands, to begin executing their plan even if they weren't fully prepared.
The issue was that Evan's own plans and the pieces he wanted in play were not all ready yet, so it would be disastrous for him if the First Finger acted prematurely.
That was why he hadn't killed Seith.
