The Marvel Cinematic Universe is full of villains and heroes with tragic pasts that ultimately makes them who they are. Whether it be an intentional or incidental death in their family or a personal tragedy, grief is usually the driving force behind the ascension of a hero, or the slide down into depravity and grief that leads one to villainy.
The defining and sometimes deciding factor of what makes one good or bad in the MCU is whether or not the character lets their grief define who they are, or, whether they use their grief as a lesson, they wish to prevent others from having to learn.
Spider-Man
Peter Parker was raised by his aunt and uncle after the unfortunate deaths of his parents. As a teenager, he was often picked on for being weak and regarded as nothing more than a nerd by bullies. Until one day being bitten by a radioactive spider, making him the wall-crawling hero we hold dear. After using his powers as a way to make money as a wrestler, Peter, upset about the amount of money he was given by the owner lets a burglar go. That same burglar goes on to kill his Uncle Ben.
Erik Killmonger
Erik is the son of, N'Jobu, a Wakandan prince who was killed by his brother, King T'Challa, after he attempted to kill Zuri for outing his plan to use Wakanda's advanced technology to assist people of African descent from oppression around the world. T'Challa leaves Erik to fend for himself in Oakland, California, resulting in him growing up resentful and out for revenge for the murder of his father by T'Challa. Now as an adult and a former Navy Seal, Erik does mercenary jobs for money, waiting patiently for his opportunity to exact revenge on those who wronged him.
Nebula
Growing up as a daughter of Thanos can't be a fun experience. Especially when he favors his other daughter, Gamora over Nebula. When Nebula was an infant, Thanos killed her parents, raising her as a merciless assassin that is blindingly loyal to him. Gamora and Nebula were always placed against each other as a way of testing their abilities.
Each loss to Gamora, resulted in Nebula being subjected to operations to upgrade her capabilities to make them equals. Nebula is forced to wear cybernetic replacements after Gamora knocks her off a cliff during a training exercise.
Drax The Destroyer
Ever wonder how Drax gained the infamous title of Drax The Destroyer? Well, Drax was once married and had a daughter, until Ronan The Accuser working under the orders of Thanos split Drax's home-world into two groups, with one to live and the other to die to bring balance to the world. Drax was placed on the side that lived but his family wasn't so lucky. Seeking revenge, he traveled across the galaxy murdering those responsible for his family's death. Eventually, he was arrested by Nova Corps and charged with twenty-two counts of murder.
Janet Van Dyne
Janet was a S.H.I.E.L.D. agent code-named the Wasp and, also, the wife of Hank Pym, the original Ant-Man. In 1987 Janet Van Dyne attempted to sacrifice herself to keep a soviet ICBM from going off, which would have resulted in the deaths of millions of American lives.
Though in reality, she had descended into the Quantum Realm, a place where space and time no longer exist. Believing his wife was gone forever, Hank retired his moniker of Ant-Man and focused his time and attention on his life with his little daughter.
Valkyrie
Brunnhilde was the leader of Odin's many Valkyries, a group of elite female warriors who were protectors of Asgard. When Hela, the daughter of Odin escaped her prison, it was the Valkyries that were sent to retrieve her. Sadly, Hela overwhelmed the Valkyries, killing all but Brunnhilde. Distraught by the loss of her sisters and angry that Odin would haphazardly send her sisters to be slaughtered by his daughter, Brunnhilde exiled herself to the planet Sakaar, changing her name to Scrapper 142 and serving under a new master as an acquisition specialist.
Tony Stark
The playboy inventor of the Iron Man suit had a home-life that wasn't all that glamorous. His rocky relationship with his father caused Tony to feel a great sense of loss. Primarily raised by his butler, Edwin Jarvis, who took on the responsibilities of the father-figure role his real father never would. As his intelligence grew, Tony took on a superiority complex, making it all that more difficult for him to take his father's belittling. In 1991, his parents went on a trip they never came back from, forever making it impossible for the two to make amends.
Doctor Strange
Like Tony Stark, the brilliant Stephen Vincent Strange was also enamored by self-grandeur. As a child, he witnessed his sister's drowning after she succumbed to a bout of cramps while swimming. Her death resulted in him swearing that he would do for others what he couldn't do for her. As an adult and one of the top neurosurgeons in the world, Strange grew over-confident and brash making it hard for others to get close to him. After a car accident that crushed his hands, resulting in permanent nerve damage, Strange's career as a neurosurgeon come to an abrupt end.
Scarlet Witch
As a child, Wanda and Pietro Maximoff were in the center of a civil war in their home country of Sokovia. One day, while having a family dinner, a mortar struck their apartment, killing their parents and trapping them beneath the rubble for two days. Knowing Stark Industries was behind the weaponry used to kill their parents, the siblings became activists against the company and later against the Avengers. So when the HYDRA scientist List enlisted them to take part in an experiment to gain powers to fight against the Avengers, the siblings adamantly jumped at the offer.
Black Widow
Recruited as a child by the KGB, she was indoctrinated and trained at the Red Room Academy to be an agent for the Kremlin. During her training, she gained the reputation of Black Widow, as she became one of the world's greatest assassins. To complete her training, she was forced to undergo sterilization as a way to end any personal distractions from the mission. During one of her missions, she was forced to execute an innocent man, feeling empathy for the first time and leaving her traumatized for the rest of her life.