In act 4, Diablo, who so far has been mostly known for chilling one-liners, engages in villainous gloating that does some damage to his image, especially if the player is cutting through the obstacles in his way as quickly as Diablo can describe it. Why such an annoying personality change? He has absorbed Azmodan. It would also explain why he absolutely loses his shit after you escape the Realm of Terror, something no being has ever done. He's not just Diablo anymore, which may interfere with his supposed inability to feel fear.
Malthael in Reaper of Souls had done unwise things for the Archangel of Wisdom: namely absorbing the Black Soulstone into his body. It seems shocking yet players forget that in the Sin War Trilogy Malthael is called the Archangel of Death, not Wisdom. He may have claimed to be Wisdom to his followers, but he was always Death. It explains why he was called to by human souls. As Death the he's drawn to souls. This also explains why he couldn't find Wisdom in the Chalice of Wisdom after the Worldstone was destroyed: Tyrael was wiser than Malthael so it abandoned him just as the dead Angels in the Halls of Justice later lashed out at Tyrael in Storm of Light. As it saw Tyrael as more worthy of its Wisdom the Chalice of Wisdom gave him the answer he sought.
If Tyrael is the true Aspect of Wisdom than who's the true Aspect of Justice!?! Imperius of course! The End of Days prophecy says that Valor shall turn to Wrath and judging from Imperius's attempts to hide his true wrathful personality until provoked by Diablo which is a cowardly thing to do he was always Wrath instead of Valor(which is just another name for courage). What is Wrath (not to be mistaken for the Deadly Sin of Anger)? The divine hand of Justice!
It seems odd that Barbarians cannot carry two handed weapons with one hand like the Crusader can, but then you realize that the Barb-exclusive mighty weapons are simply exclusive to them because they are said to be far too heavy for anyone else to carry, crusaders included.
Greed, Baroness of the Treasure Realm isn't a Sin Lord according to Word of God so why is she an embodiment of Greed? Simple: It's because the Deadly Sin is called Avarice, which is more specifically tied to money rather than treasure or other material possessions like Greed is, and as such the Sin Lord would be the Sin Lord of Avarice not Greed. The Sin Lord of Avarice's realm is likely filled only with gold and no other treasures and thus would be less impressive than the Treasure Realm of Greed, Baroness of the Treasure Realm.
Azmodan is supposed to be the best commander Hell has ever seen, the equal of Imperius even. So why'd he put in such a poor showing? Because he's a battlefield commander. He does best when he's personally directing his troops, but against the Nephalem he hid out in Arreat Crater like a pussy and just relied on their overwhelming numbers to try to win, rather than strategy. All things considered, Azmodan did fairly well as a commander—he had laid siege to the fortress, forcing the humans to meet his forces in battle, slowly depleting their manpower while he can always bring in more from Hell. Yes, he tells the Nephalem about the breach in the wall, but only does so AFTER it's happened and his forces have slaughtered almost everyone inside till they reached the hub floor where the civilians are located, which is the sin of Pride. He manages to completely decimate their stocks of provisions through Ghom, which would've broken the siege in short order if not for the Nephalem. Also, though it translates poorly in the game, Leah's journals in Act III (which can only be acquired bit by bit as you progress) imply that the entirety of the act is taking days, as she claims that she has had several times of rest in between bouts of wrestling with the Black Soulstone, times in which Adria kept an eye on it. It only SEEMS like Azmodan's a terrible commander because the game doesn't really have a noticeable day/night cycle and because the player is a literal One-Man Army. And concerning his luring Leah and the others to him (which ultimately lead to his own downfall), while also being bragging to the facet of Diablo within her, is also his gambit at having the Black Soulstone brought to him where he can seize it without too much effort (or so he thought). He COULD crush the fortress and begin scouring all of Sanctuary for the Soulstone, but it is far easier to lure its retainers to him. Remember, he is the Lord of Sin, which includes the sin of Sloth.
Another possibility for the dissonance between Azmodan's reputation and his performance on the battlefield could be that he is accustomed to fighting in the Eternal Conflict against Angels. Both Angels and Demons are functionally immortal (reforming in one shape or another in the Heavens or Hells) and fight on the Battlefields of Eternity. Azmodan's tactics are based on how to throw nigh-infinite soldiers with no fear of death or pain against similar foes, not fighting limited numbers of death averse mortals holed up in a fortress.
Also, keep in mind that Azmodan is used to fighting the Angiris Council. A group who, from what we know, relies on fairly rigid tactics and battle styles. They fight the same way each time, whereas humanity is more unpredictable (relying on more fluid tactics, willing and able to break ranks and reform to suit the situation, etc). It's the same issue that the Redcoats had in the Revolutionary war. An opponent who, plain and simple, doesn't folloe the same rules. Plus, the humans have one ace that Azmodan couldn't account for. The Nephalem. An utterly unpredictable wild card who is able to bulldoze a bloodstained swathe RIGHT through his forces and outmaneuver counterattacks before kicking his front door in and perforating his face.
After Auriel, the aspect of Hope, is captured by Rakanoth (Despair), it saps the optimism not only from the angels, but all the mortals as well. The only ones not affected are the PCs because they all have Nephalem power and are immune to the effect, Myriam due to her prophecy powers giving her time to prepare, and Shen- who has it for apparently no reason, but does seem to give more evidence to him being a former god.
The Nephalems have low opinions on Angels, turns out, it's more than just them being unhelpful. After seeing the results of Eternal Conflict, they realize it's fundamentally useless and only leaves destruction. Then it dawns to you that throughout the backstory, the Eternal Conflict was meant to be a contest to see who rules all of creation after Anu and Tathamet were gone. So basically, the Angels and Demons have been playing a tug-o-war to show who's superior and deserves to rule creation, instead of protecting the residents of said creation like humans... and from the look of it, Imperius (the jerk) and Malthael (the gone-crazy) are still all about "We will win the Eternal Conflict!". It's no wonder the Nephalems see Angels like that being no better then Demons: their concern is quite similar to the Demons, obsessed in ruling and conquest instead of protection and preservation, and the only way peace will come is if they abandon the Eternal Conflict.
Why are the Demon Hunter's resources called Hatred and Discipline?
When you can't fire your secondary attack or Archery skills anymore, you need to wait a while and fire your primary attack so that your skills can be used again. Having to use a low-damage attack can make you long to turn those big bad demons into Swiss cheese, so after using your weaker attacks you take all that pent-up hatred and let loose with your guns.
Discipline recharges much more slowly than Hatred, not even being rechargable through firing; thus, you need to be conservative with your defensive skills. Losing access to those skills from overuse shows that you lack the discipline to be careful with them.
How could a male Wizard learn from the Sorceresses? The Zann Esu are a clan of exclusively female spellcasters, so how could a male Wizard have learned from them? Simple; he specifically learned from Isendra, a rogue Sorceress who wouldn't have nearly as many compunctions about taking a male apprentice as other Sorceresses would.
The High Heavens is lead by a commander whose main strategy is Leeroy Jenkins on the battlefield and invokes Murder Is the Best Solution in a setting where Death Is Cheap for the Demon Lords (Imperius) while the Burning Hells have a commander who suffers from all seven deadly sins (Azmodan) while suffering from constant infighting among the chain of command (The bickering between the seven evils). No wonder the Eternal Conflict is named as such. Both sides are woefully inept at keeping the advantage in the long term for any meaningful progress to occur. Not surprisingly, this has lead to disillusionment between both sides alike, which will lead to the creation of Sanctuary and humanity.
Captain Rumford died on the path to the Cathedral, which means that was where most of the cultists' forces came from. Story-wise, you could have saved multiple characters if you hadn't gone straight for Wortham, which was in the opposite direction. Gameplay-wise, you can't do anything about this, but it's horrifying to realize that most of the bad stuff that proceeded to happen could have been avoided if your character took one last trip to the Cathedral.
In the larder of Bastion's Keep, you find Ghom's journal, which reveals that not only has he been eating people, but he's also been slaughtering people and feeding them to the unsuspecting denizens of the Keep. The hero and follower don't react to this, but unless they haven't been eating anything at all, they've been eating what everyone else has.
Not that it makes it any less horrifying, but that's NOT what he's been doing at all. His journal actually talks about how he's been feeding his human prisoners and a case of in-universe Fridge-Horror is delivered from said prisoners who were starting to realize that they were only getting fed fresh meat AFTER one of them had been dragged away screaming.
Sometimes you can find healing wells in the Halls of Agony, Leoric's personal torture chambers. Why are they there?
And let's not even get into how BIG the Halls are... literally hundreds of jail cells and torture devices, with multiple floors delving into the earth. Just how many people did Leoric's guards drag down here?!
So, Tyrael destroyed the Worldstone, and Nephalem powers start to return to humans. Possibly, to all humans. And many cases of Humans Are Bastards reside in the world. Imagine thousands of physical gods, good, evil, selfish, altruistic, fighting each other, Heaven or Hell... Oh, Crap! doesn't even begin to describe this.
As Tyrael says in the end, You the player character, if you become corrupted by evil, are the greatest threat to all of the realms. So, there is a possibility that you, as humanity's greatest champion, will become the one that will doom all of creation.
It's stated that the Reapers attacked just about every Human city on Sanctuary, with Malthael using the Black Soul stone to directly steal their souls later. While the Nephalem stopped him as quickly they can, Sancutary's population has dropped by a very high margin.
Tyrael is doing his best to try and get Humans and Angels to work together against Hell. This would already be hard enough, given how Angels dislike anything Demonic, while most Humans believe Angels and Demons to be fairy tales. By having an army of angels fly into Sanctuary and begin exterminating everyone in sight, Malthael gave Humanity a nightmarish first impression of Angels, and bombed Tyrael's diplomatic efforts into oblivion, likely for the rest of humanity's existence.