This is about who Mahou ends up with at the end of the story (SPOILERS FOR THE END OF THE LIGHT NOVEL MANGA AND ANIME) you have been warned

I agree that death threats etc. are too supreme, but I also get it why people are so angered about this outcome.

From my point of view you miss the point why fans are so angry about the whole situation:

1. If author planned to make it Maou x chiho ending from the start - he should have developed her well enough for people to actually accept such ending, but what did fans get? Mostly Mary Sue situations or Chiho being useless and being a 3rd wheel to the story going on about Maou how much she loves Maou and her jealousy when Emi was involved.

2. Introducing Alas Ramus and making Maou and Emi as her parents... I think I don't need to add more except the fact that if it was never meant to be real in the end, then why even introduce it in the first place. Now it just looks like a father cheated on mother and abandoned their child, really...

3. Emi and Maou character and relationship growth just to be thrown into a ditch. Simply put author wrote himself into a corner by developing their relationship, their character development and maturity into accepting each other despite being a hero and a demon.

4. Chiho won cause author was simping for her (in interviews etc. so everyone knew about it), but no one expected he would literally throw all the development between Maou and Emi out the window, for the sake of making his ship a reality. Let's be honest if he knew he was gonna do it he should have given more screentime to Chiho and develop her instead of Emi rather than make 20 volumes focus on Maou and Emi's growth, their relationship and parenting Alas Ramus. In other words he betrayed the fans who I assume mostly kept reading it with expectations of different outcome and ending which was built upon for 20 volumes+.
He could have given them some sign earlier so the more hardcore fans could drop the story/books before they get milked by buying the books for the outcome they never wanted to see, and author did it for entire 20 books just to destroy them in last book and make them despise the fact that they even supported this series by buying all the books.

There is propably more points, but I think those 4 are the biggest ones and even I agree with them. I personally don't like the story cause of the points above, I don't hate Chiho, but I would never consider her end-game girl in this story cause of how little development she got and it was never build upon. Even Suzuno got more development so her ending would be more acceptable I guess, but it would also not fit cause of how the story was written and who was the focus of the entire 20 books. Author destroyed the story himself that's the truth, he destroyed fans expectations and fanbase this series had. To avoid it he should have literally drop some hints in the story just like author of Quintuplets did (Miku and Nino were fan favs and yet they lost to the most unexpected one - but there were hints since the beginning so fans aren't that angry about it nor send death threats).

Cause of all of this this series lost huge fanbase and hype around it. Just looking at s1 viewer numbers and comparing it to s2 and s3 already shows how big of an impact this ending had and it's only my assumption but people who watch s2/s3 are ones who are indifferent to the ending / Chiho shippers or ones who didn't hear about the ending yet.