The anger is justified. But some of you need to know the context to understand better.
Diablo 3 was announced in february 2009.
It was released in may 2012, so over three years after the reveal. And it sold like hot cakes, with probably over 50 millions sales by now.
It's expansion was released in march 2014.
At Blizzcon 2017, a lot of people were expecting a D4 reveal. It was three and a half years after the latest major content updates, more than enough to have at least some content to show. Besides, 2017 was the 20-years anniversary of the Diablo franchise. The community had spoken for some time on the Blizzard forums and there was a general consensus, what we wished for was D4, or a remastered D2.. And what we got was a new DLC chraracter for 15€, and a "remastered", deliberately pixellisated Diablo 1 that, due to some bizarre thought process, would be alaivable only one month per year. There was a lot of outrage over this deliberate choice of not giving what the community wanted.
Also, anger at the incompetence of Blizzard's leadeship. You just don't put aside a license with that kind of sales numbers. Even morons should be aware of it. It is like... like... The Game of Thrones production being paused after season 3 for a few years because the boss want the team to work on a smaller project, for God's sake!
Now comes Blizzcon 2018. A lot of us hoped that D4 was now advanced enough that we'd have at least some videos to wait the 2-3 years between reveal and release. Surely they had learned from what happened last year, and knew what the community expected, right?
And we get a "new" mobile game that no one wanted. A "new" game made of recycling old D3 assets, with not even a single new character. The graphics of D3, the gameplay of D3, only a few new bosses, and the limitations of a mobile game.
What about PC players? What do they get for their patience with the lack of major content since 2014? A slap in the face, pure and simple.
And the QA following this reveal is making things worse.
Blizzard: "We know what the community want!"
Public: "Can we play this Diablo Immortal on PC?"
Blizzard: "No"
*Public start booing*
Blizzard: "Do you guys not have phones?"
Public: "What is next for the Diablo Franchise besides this mobile game?"
Blizzard: "We have several projects".
Public: "..."
Add to that Blizzard trying to censure the negative feedback.