When a supervillain is deceptive and treacherous, you're supposed to hate him, not adore him because Marvel Cinematic Universe.
Charming as Loki might be in the MCU, it still doesn't mask some of his most atrocious deeds. Throughout his lengthy stay in the several phases of MCU movies, Loki has done plenty of bad, and has only shown glimpses of true benevolence. He's the god of mischief, after all, and he has to live up to expectations. Has he?
TRANSGRESSIONS AGAINST THOR
Well, no surprise here. Loki and Thor have always had a love/hate relationship — though it gravitates more toward the "hate" part. Loki loathes Thor and thinks him an oaf, partly because he's jealous of his big adoptive brother and partly because Thor tends to be a brute. As such, Loki has betrayed Thor and his adoptive family many times.
Of course, Thor often got the brunt of it. Loki even ordered Thor to be killed as early as Thor's debut in the MCU. On top of that, there are also lies, hurtful words, and heartbreaks that he inflicted on Thor. So, it's basically a normal sibling relationship that they portrayed. Oh, and Loki is never apologetic about his violence against Thor — siblings through and through.
FAKED HIS DEATH
No matter how many times Loki hurt Thor and vice versa, both of them still love each other like family and blood. Hence, Thor became devastated when Loki seemingly died near the end of the Thor: The Dark World. They did share a meaningful childhood despite growing apart.
Then, as it turns out, Loki merely faked his death. He was rather casual about it and didn't care much about how the rest of his family would take the news. Thor eventually did find out and confronted his brother but Loki didn't seem to be moved by all the emotions Thor poured on his lap, even if Thor did mourn his fake death.
USURPED ODIN
One of the reasons why Loki faked his own death was to seize a great opportunity, and that is to sit on the throne of Asgard unchallenged...as Odin, his adoptive father. Loki revealed this at the end of Thor: The Dark World. The worst part was that he cast a spell on Odin and exiled him in a retirement home on Earth.
For a time, Loki ruled the Asgardians in their process of recovery from the Dark Elves' attack. Leaving your father in a retirement home in order to claim his throne and fool your very own people is one of the greatest acts of deception carried out by Loki in the MCU — even if it was short-lived.
HELPED THE DARK ELVES
Before he even set out to pose as Odin, Loki had already been mobilizing against his adoptive father as an act of revenge. While Loki was imprisoned in Asgard's dungeons, the Dark Elves suddenly invaded Odin's palace in search of the Reality Stone and came upon Loki. Seeing this as an opportunity to get back at Odin, Loki pointed the Dark Elves in the right direction.
His intent was to have the Dark Elves carry out the revenge against Odin, but his plan backfired as the Dark Elves killed Frigga instead, Odin's wife and Thor and Loki's adoptive mother. One would think that Loki would've learned a lot from this but he kept on being his usual self after a short bout of guilt and anger in his cell.
KEPT STEALING THE TESSERACT
The god of mischief was one of the first recurring faces of villainy in the whole Avengers franchise. He proved to be a successful villain for the first Avengers movie and an effective one at that thanks to his penchant for stealing one of the most powerful objects in the MCU, the Tesseract — otherwise known as the Space Stone.
Loki knew full well the repercussions of his actions and what calamities would happen if he were to steal such an object. Throughout all the MCU movies, Loki has stolen the Tesseract at least thrice, each in three different movies. He just doesn't know when to give up.
KILLED COUNTLESS INNOCENTS
As soon as Loki realized that most of his life was a lie, he immediately embraced his evil tendencies. The fallen Asgardian quickly upped his notoriety by killing countless innocents on Earth — and probably other planets as well. We even get to have a look at his evil acts in the first Avengers movie.
Loki harmed, tortured, and murdered many in the process of finding the Tesseract. In addition to that, he also put his homeworld of Asgard in constant danger whenever he was around. Odin's words about Loki being followed by death and destruction wherever he goes definitely rang true.
INVADED EARTH
In addition to murdering and planning to enslave the human race, Loki also unleashed a horde of bloodthirsty Chitauri as his army during his invasion of Earth. This lead to the Battle of New York in the first Avengers movie and the culmination of all of Loki's plans ever since he left Asgard to become a villain.
Despite the invasion not quite reaching a global level, Loki still managed to destroy and traumatize a huge chunk of New York City. It's also logical to assume that humans were killed during the Chitauri onslaught given all the explosions and the collateral damage.
TRIED TO TAKE OVER ASGARD
Posing as Odin wasn't the first time Loki tried to take over Asgard, of course. This happened way earlier in the first Thor movie where Loki took advantage of his brother's absence. Odin became weak and Loki was left pretty much unchallenged on the throne since everyone didn't yet know that he's a Frost Giant.
What happened next was disastrous for Asgard. Loki became the acting king betrayed his father and Asgard in all sorts of ways. Needless to say, he abused his power and quickly transformed into one of Asgard's most dangerous enemies.
NEARLY DESTROYED JOTUNHEIM
It wasn't just the Asgardians that Loki ended betraying but also the Frost Giants from Jotunheim, Loki's kin. Loki sicced them on Asgard to make it appear that they invaded and were about to kill Odin until Loki betrayed them to make himself look like the savior.
The worst part for these Frost Giants is that Loki also set out to destroy Jotunheim completely after killing Laufey, the Frost Giants' leader. In the process, Loki nearly obliterated all of Jotunheim if not for Thor's interference. To think that Loki will sacrifice a whole planet just to look like a hero goes to show how little shame the god of mischief has.
SERVED THANOS
All other Loki misdeeds pretty much pale in comparison to this one. It's one thing to betray your family, but betraying the whole universe just because you can is pure evil. Loki consciously served Thanos with the initial goal of becoming "king" of the Earth. Their deal involved Thanos giving Loki the Mind Stone-infused scepter in order for him to find the Tesseract.
Of course, Loki failed to do this and basically handed the Mind Stone over to the Avengers. Despite the strategic gain, Loki still caused plenty of harm and devastation. Besides, you're pretty much g your own death sentence by making a deal with the Mad Titan. It seems Loki got his just desserts on that front.
