The ultra-powerful Scarlet Witch has long been blamed for creating the horrifying House of M reality, but in truth, it was an entirely different Avenger who manipulated Wanda Maximoff into using her reality-altering powers to create a violent and dystopian new world order. While he might have been doing it in an attempt to save his mentally ill sister, Pietro Maximoff, also known as Quicksilver, is the true mastermind of the warped world and should be the one taking the blame for the House of M.
House of M was an event that spun out of "Avenger's Disassembled," which saw Wanda Maximoff losing control of her reality-warping powers after realizing that her children were fake, made from her magic and parts of Mephisto's soul. This realization ends in her brutally murdering three Avengers, Ant-Man, Hawkeye, and her husband, Vision. After this event, the Avengers and the X-Men got together to make a decision on what to do about Scarlet Witch, but before they could even find her, the world was rewritten by Wanda's powers. The House of M reality was born, an Earth where Xavier died and Magneto become the leader of the world-dominating "House of Magnus" after mutants finally became the "superior species" on the planet.
To be clear, the tragedy that was Decimation - also called M-Day - which occurred after House of M and led to the disempowerment of 99% of the world's mutants was 100% entirely the Scarlet Witch's doing. Wanda willingly chose to utter the infamous words, "No more mutants," without anyone influencing or manipulating her. However, it is not fair to blame Wanda for the events of House of M #1 - illustrated by Frank D'Armata and Olivier Coipel with writing by Brian Michael Bendis - because even though it was her powers that caused the shift in reality, it was her brother Pietro that convinced a clearly unstable and mentally ill woman that this was a good idea.
It is revealed in House of M #7 that the X-Men and Avengers were planning on killing Wanda as she was proving to be too uncontrollable, so her brother begged Magneto to help. But their "father" refused, shouting, "What would you have me do?!" Pietro, unwilling to accept Wanda's death, convinced her to use her powers, telling her, "You could take it back. You could make... everyone happy," before manipulating her further by telling Scarlet Witch if she doesn't do this, they will never get to be together again.
When Pietro's manipulation and lies are finally revealed to Magneto, he brutally attacks and kills Quicksilver by crushing him with metal after screaming, "You used her and you used me! I would never have allowed this! You've destroyed everything and everyone!" Finding her brother's dead and mangled body is what pushes Wanda over the edge again, leading to her utter the "No more mutants" spell that she is so well known for.
It can be argued that Magneto is actually the one behind all of this because of his years of abuse and emotional manipulation of his two "children," but Pietro must be held accountable for his willful scheming and the way he manipulated his sister, even if he was trying to save her life. Regardless of how the world views Scarlet Witch for causing M-Day, it is quite clear that Quicksilver was the mastermind behind the House of M event and Wanda Maximoff should be forgiven for this misuse of her powers. Scarlet Witch was just one more victim of the selfish decision that the distraught Quicksilver, a former Avenger, made which led to the dystopian House of M reality.