Quicksilver, aka Pietro Maximoff, was a founding member of Magneto's Brotherhood of Evil Mutants and joined the Avengers in their early days, becoming apart of "Captain America's Kooky Quartet" alongside Hawkeye and Scarlet Witch, Pietro's sister. He was an on-and-off member of the Avengers, and later married and had a child with Crystal of the Inhumans.
Quicksilver and Scarlet Witch are believed to be children of Magneto, and, if so, Pietro certainly inherited his father's ego and flexible morals. Pietro has caused his fair share of disasters over the years, and those disasters are what we're going to look at today.
10 Stealing Terrigen Crystals
After the events of M-Day and Decimation, Pietro Maximoff was desperate to undo the damage he caused by coaxing Scarlet Witch into creating the House of M. He was taken to Attilan after committing suicide, and he came up with the idea of using the Terrigen Crystals to restore the power of mutants who lost their abilities on M-Day. He asked Black Bolt for permission to use the Terrigen, but the king said no. Not one to take 'no' for an answer, Pietro stole the sacred Terrigen Crystals. This is but the first of many sins Pietro committed during this period that will be included on this list.
9 His Treatment Of His Wife
After a battle between the Avengers and the mutant-hunting Sentinels, Pietro was badly wounded. Crystal of the Inhumans took Quicksilver back to Attilan for treatment. He was there for months, and he and Crystal fell in love with one another. They would soon marry, but Pietro proved to be neglectful husband--a situation was worsened when Maximus the Mad took control of Quicksilver for an extended period and turned him against the Inhumans, the Avengers, and the Fantastic Four. Even after Pietro was freed of Maximus' control, he and Crystal would be separated for a long time before finally getting their marriage annulled.
8 Joining The Brotherhood Of Evil Mutants
Pietro's first sin was joining Magneto's crusade as part of the Brotherhood of Evil mutants. This initial team consisted of Magneto, the Toad, Mastermind, Quicksilver, and Scarlet Witch. They would do battle with the X-Men, and they would lose in this confrontation. After this initial loss, both Wanda and Pietro would leave the team, joining the Avengers before too long.
7 Trying To Kill Layla Miller
During the post-House of M period of Quicksilver's attempts to make up for his mistakes, Quicksilver took up residence in Mutant Town, a poor section of New York primarily inhabited by mutants.
In his growing madness, he kidnapped Layla Miller-- a member of X-Factor Investigations and the woman who restored many heroes' memories during House of M--and tried to kill her. He claimed she was inhabited by the devil, but he couldn't bring himself to follow through with killing her.
6 Killing An Inhuman
When Quicksilver first exposed himself to the Terrigen Mists after House of M, he found himself able to time-travel. Pietro used this ability to team up with iterations of himself from different points in the time stream, and he went back to take the Terrigen Crystals. One Pietro took the Crystals while the other stood watch. An Inhuman guard named Goran tried to stop Quicksilver, and Pietro accidentally killed Goran while defending himself.
5 Manipulating His Daughter
When Pietro initially manifested his obsession with the Terrigen Crystals as a means of restoring mutant powers, he needed help in obtaining the material. He and Crystal's daughter, Luna, proved an ideal aid in this task. Luna wanted to get to know her father better, and Pietro used this to get Luna to help him steal the Terrigen Crystals.
4 Using Terrigen On Mutants
Even after stealing the Terrigen Crystals and escaping Attilan, Pietro's mission turned into a dismal failure Whenever he used the Terrigen on mutants, it would have disastrous side-effects. It would often restore powers, but it would just as often kill or maim mutants. Despite this, Pietro's obsession kept him going, even opening a "clinic" in Mutant Town to "restore" mutant powers with the Terrigen.
3 Causing The Silent War
Quicksilver's robbery of the Terrigen Crystals also led to a conflict between the Inhumans of Attilan and the United States. O.N.E. helped Attilan track down Quicksilver, and O.N.E. was able to get their hands on the Terrigen Crystals.
However, the United States decided not to return the Terrigen Crystals to Attilan, and this led to the Silent War. The endpoint of the Silent War leaves Attilan in ruins and Maximus the Mad in control of most Inhumans of the city.
2 Getting His Daughter Addicted To Terrigen
On top of everything else Quicksilver did after House of M (told in the Son of M story), Quicksilver also forced Luna to expose herself to the Terrigen Mists. Through repeated exposure, thanks to Pietro having Luna help him procure the Terrigen Crystals, Luna developed a drug-like addiction to the Terrigen. In the end, this was one of the things that broke Pietro out of his obsession and let her return to Attilan.
1 House Of M
The original sin that led to most of the above-listed crimes of Pietro Maximoff was House of M itself. Pietro feared that the Avengers would execute Scarlet Witch after the events of Avengers Disassembled, and he coaxed her into creating a world where mutants were the dominant species and humans were hunted and oppressed. This was a vile manipulation of Wanda in her mental instability, and it almost left the world in ruins. It also left mutants a further endangered species after the infamous "No more mutants" moment.