According to the WoW comic (which most certainly is canon lore), C'Thun is dead. If you've done the encounter, you'll see that we didn't just fight his eyeball. That was only one aspect of the fight. We also go inside of C'Thun, and mangle his organs. We destroy his stomach from the inside out. I'm not a doctor or an Old God physician or anything, but I'm pretty sure that most things die when their stomach ruptures. Or is riddled with holes from swords and such.
A lot of people will use Herald Volazj's quote to combat this, where he says, "They do not die; they do not live. They are outside the cycle." It's often used as evidence as to why C'Thun can't possibly be dead. Me? I don't think that's referring to the Old Gods. I think it refers to the Scourge, but I've discussed that before.
C'Thun is dead, and it has been stated as such both in-game (quest text) and in the WoW comic (narrator) so he's pretty much dead. We gutted him from the inside out. Kil'jaeden? Most certainly not dead. Where Blizzard has clearly stated C'Thun is dead, they've done the exact opposite for Kil'jaeden: They have said that he is not dead, and his death animation is a clear indication that he's not dead. He leaves behind no corpse. He just falls back into the Sunwell, which he was using to enter the world.
Why do Goblin Sappers blow themselves up on purpose when they have nothing to gain from it? They aren't exactly a race of religious zealots.
Some Goblins are frighteningly intelligent. These Goblins are their tinkers. Some Goblins are frighteningly stupid. These Goblins are easily manipulated by smarter Goblins and become Sappers.
Sorano asked a few questions...
Are the Naaru the source of the Holy Light or simply very powerful avatars channeling it?
A little of this, a little of that. We don't know the exact nature of the Naaru, but we do know that they are not the ultimate source of the Light. They can serve as sources of the Light, but they aren't the ultimate pinnacle of the Light, they didn't create it. To try and draw a real world comparison that is likely horribly flawed but I'll use anyway, picture the Naaru as batteries. You can get electricity out of batteries. If you put a battery in an electronic device, it'll make that device run, but batteries aren't the source of all electricity.
The Naaru are closest to powerful avatars of Light channeling it, yes, but we've also seen that you're able to draw LIght out of them. So while I say 'a little of this and a little of that' it is much closer to that than this. Does that make sense? No? Well, I tried.
Let's just stick with 'powerful avatars' for now.
On the Collector's Edition DVD, a woman talking about Warcraft lore (her name escapes me) mentioned the source of power behind Arthas. That struck me as odd because I had never considered it before. Is undeath a school of magic in the WoW universe or simply a deeper form of arcane or shadow magic? Or could death or shadow magic simply be part of a greater force that empowers being like the Holy Light, for example?
Necromancy is a very dark, grim form of Arcane magic. Rather than shooting Fireballs or channeling Arcane Missiles or conjuring flasks of water, you're raising horrible fleshy constructs from corpses and tying someone's soul to the mortal realm. The Schools of Arcane Magic - Necromancy more or less states that Necromancy has total power over flesh, living and dead.
The big issue here is that there is no part of the Lich King that naturally knows how to wield that power. Ner'zhul was a Shaman, not a Necromancer. The woman that was talking, Evelyn Fredricksen, was mainly pointing out that the Lich King's power over Undeath is not a natural ability, it's not something that Ner'zhul could do on his own. It's a power that was given to him, and precisely how that was done is something that we might learn on our journey through Northrend. As of right now, we still don't really know the full details of what was done beyond a theory that he's tied to Yogg-Saron in some way.
Has the concept of afterlife on Azeroth or the greater WoW universe ever been explored? There are many varying religions and faiths. I know that there was mention of it in Warcraft 1, but that aspect of the lore seems to have been diminished with the introduction of WoW. Is there a heaven or hell or even multiple planes of spiritual afterlife? I even remember reading somewhere about demons, from what is referred to as Hell in WoW Lore, that were even at odds with the demons from the Burning Legion.
The concept of Heaven and Hell were very much there in the very early iterations of Warcraft, but just like you said, they're not really there anymore. Overall, it seems that they've moved away from that and intentionally leave it vague. That being said, it does seem like there's still a concept of Heaven (or Heaven-like place) for those faithful to the Light. We see followers of the Light ascend to the afterlife in a few different places in WoW, two examples being the Benediction quest and the much more recent quest chain in Icecrown with Crusader Bridenbrad. Is there an afterlife in WoW? Yes. What does that afterlife entail? We don't know. They don't tell us, and I think that's intentional. What lies beyond is supposed to be vague and mysterious.
I'd also like to note that the Spirit Healers we see in-game probably do not play into this at all. They might have a role in Warcraft's afterlife, but it's much more likely that they're just a game mechanic in case you can't get to your corpse safely and easily. It's an MMO thing.
Thanks for your time!
Ask a Lore Nerd is here to answer all of your questions about the lore and story of the Warcraft universe. From the religions of the universe to the 'evils' of Azeroth, everything is fair game. If you want more in-depth answers to some of your questions, you may find what you're looking for in Know Your Lore.