The 2005 "Decimation" event, in which Wanda Maximoff/Scarlet Witch eradicated the majority of mutants, has had a ripple effect in over the decade since it happened. The near-extinction of mutants changed everything, and House of X #4 puts that event in perspective as compared to other mutant tragedies while also revealing just how mutantkind views Wanda now.
In one of writer Jonathan's Hickman's oft-used infographics, the issue reveals "a list of humans who have committed major mutant crimes." Wanda is ranked second, with her Decimation having killed 986,420. She's a fair ways back from Bolivar Trask, the creator of the mutant-hunting Sentinel robots responsible for 16,521,618 deaths in total. However, she's well above such leaders of hate groups and/or mutant massacres as William Stryker (414), Donald Pierce (348) and the Leper Queen (221). Still, she's the only person on the list who has been a major hero in the past.
Gone mad with grief over a flood of memories from her traumatic past, Wanda lost control of her reality-altering powers during a cascade of massive crossover events. Avengers Disassembled saw the death of multiple characters and the disbandment of the team, House of M saw Wanda create an idyllic alternate universe where mutants reigned as the majority and ultimately Decimation concludes the event by nearly erasing the X-Gene from existence. Many mutants lost their powers, and for years it was believed no new mutants would be born.
The numbers attached to the Decimation have always been erratic, ranging from 16 million purged mutants to 1 million. Even the follow up comic The 198 Files, meant to detail the experiences of the sole 198 remaining mutants, explained that the 198 number was merely an estimate. House of X more or less corroborates those numbers, though giving them a more definite total of just under one million. with the total percentage of mutants falling drastically in kind.
The Decimation is described as "mutant erasur [sic] by the Pretender Wanda Maximoff." The title, and Wanda being called a human in general, seemingly refer to origins of her and Pietro Maximoff/Quicksilver getting retconned, making it so she didn't actually have an X-Gene but instead gained powers from the DNA-altering High Evolutionary.
The retcon was only the latest in a long line of alterations to Wanda's past, which has changed a fair bit over the years. While the character debuted as a reluctant member of the Brotherhood of Evil Mutants, she soon after left the group for heroic pursuits in "Cap's Kooky Quartet" team of Avengers. She became a mainstay of the group thereafter, but that didn't mean her relationship with the X-Men was over.
Although Wanda and her twin are best remembered as Magneto's children, initially that was not the case. The Master of Magnetism enfolded them into his cause after saving their lives, but in 1979 he was revealed to be their true father. That remained the case until 2014's AXIS event. When Wanda cast a spell affecting her bloodline, Pietro was seemingly killed. However, Magneto remained unaffected.
Eventually the High Evolutionary revealed the current state of affairs, that the humans initially believed to be their parents were indeed their real parents but that their powers actually resulted from the High Evolutionary's own experimentation. Although the implications of the reveal have remained underexplored, it's possible that House of X or one of the forthcoming X-titles will deal with it.
How does mutantkind view one of individuals most responsible for quelling their numbers masquerading as a mutant for so many years? Indeed, the Brotherhood was synonymous with mutant supremacy, and even Scarlet Witch and Quicksilver's places on the Avengers were partially justified as being ambassadors for their people. After the events of M-Day, Wanda was largely presumed dead, resulting in confusion as only some remember the alternate world that she created. She has attempted to atone, but the X-Men don't seem to have forgiven her.
Another haunting parallel to the Decimation event are Charles Xavier's final words at the close of House of X #4. After the tragic death of the squad of X-Men, many of whom were fixtures of the team like Cyclops and Wolverine, Xavier's reaction was one of grief. He says "No more," as tears roll down his cheek. Wanda said, "No more mutants." This very well could be setting up the next stage of Xavier's plan and some act that duplicates the brutality of Wanda's Decimation.