Note: For the other two games, see here and here.
Kerrah: How can you have a "Cracked Sash"?
Do you see anyone doing laundry on Sanctuary?
Why didn't the angels intervene during the Dark Exile or Baal's assault on Mount Arreat.
Because the angels voted on whether to destroy humanity once, and Tyrael feared that if they became aware of what was happening they'd vote to destroy humanity and the world to stop the forces of Hell from winning the war. You can thank Imperius (jerk) for that.
You can really chalk it all up to Imperius as the above troper stated. He'd rather stand aside, let the Burning Hells take Sanctuary and then have them use that to take out the Heavens than lift a finger to help humanity. It's pretty clear that Heaven is in decline since he took over so it's no surprise that he's probably the reason they were fated to be defeated by the Burning Hells.
Why is Diablo named "The Dark Lord" in game?
Hell. Not the difficulty, the regions. So far, we had the final areas in the first game, and Act 4 in the second that take place in a place called Hell, yet these are clearly physical areas, and not other dimensions. In the first game, it's understandable, because Diablo created them by warping the lowest levels of the labyrinth into a Physical Hell. In the second game, You enter hell through a portal, the same one Diablo took, implying that this IS the actual hell, but then where exactly is he banished when his soul-stone is destroyed? Same goes for Andariel and every single demon that was banished back into Hell.
The Hell you see in D2 at the end may not be the home plane. It may be a bridge dimension that allows them to move to and from the world, Sanctuary. The soulstones may be what allows them to go back and forth so freely or even use it.
Their souls aren't sent back to the Burning Hells but to the Black Abyss which is a distinct place from the Burning Hells.
Answered in Diablo 3. Diablo and Andariel's souls were sucked into the Black Soulstone.
Just...*ahem*...where did that helmet come from on that Decapitated Body...?
Maybe it was used as a weapon, or said body's owner kept it in his other hand as a memento. But is it really that important?
If the helmet was on their head then they wouldn't have been decapitated, see?
Perhaps this is the body of the Nearly Headless Nick?
Maybe the head was just nearby and you dumped the head out of the helmet before claiming it? Or maybe he wasn't wearing the helmet and that's why he lost his head?
I just had a thought. Since the current Diablo series resembles the late Middle Ages/Renaissance, and the Sin War Trilogy is the prequel, does that mean that the setting of the aforementioned trilogy resembles ancient Greece/Rome?
Where exactly did the labyrinth come from anyways? Is Tristram on top of a giant cave or did Diablo dig it out?
The former. The Horadrim found or made that cave system and sealed Diablo there, and Tristram was built on top of it.
Isn't there a trope about building stuff on top of your "defeated" enemies? (And then sometimes ending up realizing how big an oops that was...)
So what happens to people when they die?
You know, that's a very good question. According to the Witch Doctor from Diablo III, they go to some place called the "Unformed Land," though I'm not really sure about the afterlife beliefs of some of the other cultures of Sanctuary.
The Necromancers believe the dead go to the keeping of the Great Dragon, Trag'Oul, in some separate spiritual realm. Since they talk to the dead, they might be the ones to know. Ghosts obviously do occasionally appear, but they never mention and seem not to remember where they've been. It's never mentioned where the souls of angels or demons go. In any case, it's clear that humans don't ordinarily end up in heaven, but based on the monster lore corrupted human souls can be made into demons. Near the end of III, Tyrael suggests that Malthael has spent millennia trying to understand the inexplicable mysteries of life and death, so it seems that even in-universe, the answer is not well-known.
Seeing as how Trag'Oul is an actual entity expounded upon in the books we can easily assume that, if the Unformed Lands exist at all, then they're probably like a spirit realm for ghosts while the souls who pass on go to Trag'Oul. It might be similar to what happens with Wraith the Oblivion, where the ghosts who refuse to pass on live in the Shadowlands.