Sure both, Fain and Slayer are evil, and both were basically made by the DO. However, I think Fain deserves death even more than Slayer for the atrocities he did. Besides that, it seems Fain has served his purpose (at least if RJ won't be that unoriginal to give Fain a Gollum-like role in the end..). I think it wouldn't add much to the story if Fain yet tries again to corrupt a certain group to kill Rand or harm the Shadow. On the other hand, there are interesting plot possibilities for Slayer's links to people like Lan, Rand, Moridin, Farstrider or Perrin.

Mashadar and Machin Shin didn't harm Fain, because of the DO's and Mordeth's influence. But Fain isn't invincible as we see e.g. in WH when Rand wounds him. And even though Fain is of course quite dangerous, I don't seem him take out around 50 Darkhounds and Slayer with the ruby dagger or his illusion tricks (and frankly, it's really obvious now that Fain is the DH's prey, since both weren't seen since WH/CoT).

I expect Fain to die in a similar short scene like Masema had. Fain will be on the run, hunted by the DH, hearing their howls coming ever closer. In the end, Fain will stop to run and face the pack, probably take a few of the DH with him, but be ripped apart by them in the end: The DO's hound killed by the Darkhounds. Probably Slayer will observe Fain's death, maybe then musing that he finally finished the task to kill this traitor, like his previous task to kill another traitor, namely Asmodean..