When a storyline is followed up by an event called the Decimation, its a big deal. I mean lets forget that the entire storyline took place in a warped reality (not an alternate timeline, but this reality...warped), but it ended with 95-99%, depending on who you ask, of mutants losing their powers in one form or another. The truly lucky ones were turning 100% baseline human, however, a large portion of the now-former mutants were left with remnants or consequences from their lost mutations (like Marrow and Blob) and many still simply died for one reason or another (Magma's boyfriend Antonio or poor Wing). With so much tragedy also came much blame being thrown around. But whose fault was it really?
The obvious answer of course is Wanda Maximoff, the mutant Scarlet Witch. It was her powers that killed many Avengers and destroyed Avengers Mansion before warping reality into the House of M and last but not least, as she reverted the world to somewhat normal, she cast her spell to depower the mutants of the world. Whether she was in her right mind or not, it was her mutant hex and magic abilities that wreaked so much havoc upon her friends and family. But what caused her to do this?
What about Janet Van Dyne? In her own giggly drunken state she started blabbing about Wanda's lost children, causing Wanda to begin her slide off the deep end. As the Wasp, Janet has been in all sorts of dire situations and probably, I suspect, more then her fair share of parties and drunken moments. So why suddenly after a few simple cocktails by the pool could she not hold her own tongue when she's done so many times at Avengers or UN functions in the past? So maybe she gets a bit of blame of Avengers Disassembled, but did she cause House of M or M-Day?
How about Pietro Maximoff, the arrogant speedster Quicksilver? He's been an Avengers and an X-associate for many years and at the first sign of these teams thinking to harm his sister, and he runs to Daddy? A father, I might add, that he has denounced over and over through the years and he has fought against as a members of the two superhero teams. Keep in mind that while Wanda had already caused much destruction, she was now sedate and calm and no longer hurting anyone, so I don't suspect that they were ACTUALLY going to kill her at that moment. Yet, Quicksilver felt like then was the exact moment to steal his unconscious and deranged sister and bring her to his still-sort-of-evil father for help? Not to Professor X, who happened to be handy, but to the attempted world-conquering Magneto? Perhaps not the brightest of moves Pietro...
And yes, let's talk about Max Eisenhardt (aka Erik Lensherr aka the mutant terrorist Magneto). Now granted that he didn't know for many years that the twins were his children, but ignorance is no excuse for tyranny and abuse towards people that (for one reason or another) are helping you in your crusade. And even once he discovered they were his children, he never once put their needs before his or the needs of mutants at large. Even after finding out their lineage he continued he continued to crusade against their respective teams (Avengers and X-Factor) and ignore any familial responsibility or obligation he had. Even at their worst points as enemies, Mystique always cared about Rogue and Nightcrawler in her own way (and back in the 80's Rogue and Mystique had many mother/daughter moments in between battle).
There's been much debate and controversy as to who is truly responsible for M-Day, and the truth is that its everyone and no one's fault. Each of these people (and many other, like Wolverine and the rest of the Avengers/X-men in general) had their own parts to play and it is tough to lay blame squarely on just one person. Its easy to say that Wanda went nuts and wreaked havoc on everyone around her but we would be remiss and irresponsible to ignore the parts of fault that lay on her brother, father and best friend.
So there is some food for thought, until we find out that someone masterminded the entire thing and it was really none of their faults.