I don't really like how Diablo IV tells its story via its open world quasi-MMO design, particularly with so many freaking side-quests.
You're definitely not supposed to do all or even many of the side-quests whilst following the main story, but whilst they separate the quests on the quest-window, as
@Whizbang Dustyboots says, their failure to follow WoW in using a different icon was definitely a failure of design. I think it's a different colour (white for main vs blue for side, maybe) but that's not enough.
So that is on them. The whole quasi-MMO world is the weakest aspect of D4 currently. Maybe they'll do something cool with it, and Helltides are kinda fun, but...
And this whole both hell and heaven being full of jerks is getting a bit tiring.
This has been true since Diablo 2, note, so complaining about it now is a bit late. It's literally a plot point in D2 that the angels don't have humanity's interests at heart, and that Tyrael is weird because he does (particularly that he's willing to essentially die for us). Then D3 tripled down on that 12 years ago. D4 is actually soft-pedalling it quite considerably compared to D4.
Claiming it stole it from WoW when it was doing it before WoW though is just anachronistic. If it stole it from anywhere, it stole it from
Spawn (the Image comic series), which was a major influence of Blizzard in the later '90s.
And Blizzard is doing much the same in WoW, where the Titans, Naaru, and nearly everything else previously good-aligned cosmic forces are being depicted as jerks in a sort of false equivalence both-sides-ism, which has increasingly grated on my nerves and grown tiresome for me.
You're entitled to be mad about this if you want, but lay the blame in the right place, please. This is absolutely classic Gen X stuff that was downright common in the 1990s. It's not "increasingly true" or anything. It's always been a significant part of WoW. The Naaru are a particularly bad example for you to use, as they were literally painted as unreliable fanatics who sometimes turned darkside when they were introduced in The Burning Crusade. Anyone who saw them as "good guys" or "good-aligned" simply did not understand WoW's lore, and frankly, WoW's lore at that point was not very complicated. I remember this particularly because I was in a quite lore-oriented guild in TBC, and this was a frequent topic of discussion - absolutely no-one trusted the Naaru, those big crystal creeps.
The Titans too were never "good-aligned". Where did you get that idea? Order-aligned, sure, but it's a big mistake to see order-aligned as "good-aligned". There's never been any real evidence that that Titans had the good of the peoples of Azeroth in mind (in general, there have been individual exceptions), and tons of evidence that they'd happily genocide the entire planet if they felt it was "necessary" - and nearly did.
Again this stuff isn't new - it's been true since WotLK at the latest. What's new is that because the Titans are "back in the news", people are having to realize "Oh actually they were never good guys", and because some people have (bizarrely imho) been apparently thinking they were for 10+ years, they're reacting badly.
Now that Metzen is back, I suspect that's going to be dialed back some. The edgelord stuff seemed to be the old regime that was shown the door during Shadowlands for all of their gross behavior.
Nah. Metzen was behind most of the "neither side can be trusted" stuff, because he's a real Gen Xer in terms of media culture, and Gen X media culture holds you can never trust "either side" when there are extremists involved. Only people who aren't extremists can be trusted - only people who can let stuff go, put people first, and who won't blow up a planet just to stop someone.
The edgelord-iest stuff in WoW is all from when Metzen was still there, note. Cataclysm is wall-to-wall edgelord-ery, as is WoD - WoD has some ridiculous stuff straight out of '90s comics particularly, and was way edgier before someone at Blizzard told them to knock it off (rape plotlines etc.). Metzen quit in 2016, so when Legion came out, and thus the only expansions without his influence are Battle for Azeroth, Shadowlands, and Dragonflight. Dragonflight is easily the least-edgy WoW has ever been, including Vanilla. BfA was at most moderately edgy - nothing compared to Cataclysm or WoD - and Shadowlands has what sounds like an edgy concept, but isn't really particularly so. Again I'd say it's as nothing to the edgy-ness of Cataclysm or WoD.
EDIT - It may be worth noting Metzen has a different position now to what he had during the WoD/Legion era. Specifically, he got promoted to a position where he had less direct influence on WoW's story and writing, because he was in charge, at one remove of the story/writing of all Blizzard games - specifically "senior vice-president of creative design and story development", whereas in Cataclysm and earlier, he was straight up creative director, and he is now executive creative director (with no creative director beneath him, so he's once more WoW's de facto "showrunner"). So that may explain why WoD was so ludicrously edgy, but it doesn't really explain why Cataclysm was, unless he'd already started to take a back seat. I mean, did he really approve stuff like Garrosh calling Sylvanas "bitch"?