I have a feeling that the first time I met Jon Hawker, I killed him.
And the Tribunal ARE gods, and possibly have always been gods. As for being able to kill them:
QUOTE(Vivec)
It's nice never being dead, too. When I die in the world of time, then I'm completely asleep. I'm very much aware that all I have to do is choose to wake. And I'm alive again. Many times I have very deliberately tried to wait patiently, a very long, long time before choosing to wake up. And no matter how long it feels like I wait, it always appears, when I wake up, that no time has passed at all. That is the god place. The place out of time, where everything is always happening, all at once.
Once you have stripped them of the source of their power, and possibly doomed the entire world, the individuals are presumably mortal. Whether ALMSIVI continues to exist or not, I do not know.