Though they fight on opposite sides, sometimes there is a fine line between heroes and villains. Complex heroes can often do things the audience won't always agree with, while complex villains can be sympathetic even when carrying out their evil plans. It's good to have a little gray area to keep fans on their toes. The MCU has given us examples of both in their movies.

RELATED: Marvel: 5 Villains Who Became Heroes (And 5 Heroes Who Became Villains)

While it's always pretty easy to cheer on any one of the Avengers, some of them are more likable than others. Whether it's the things they do or the way they are portrayed, they aren't always as beloved as their team-mates. On the other hand, some bad guys are so charming and interesting, that it's hard not to root for them a little. Here are some of the MCU heroes fans hated and some of the villains they loved.

10 Hated Hero: Iron Man

tony stark showing off the jericho

To be clear, Tony Stark (aka Iron Man) is one of the most popular Avengers, and Robert Downey Jr. played him masterfully. The arrogant and brash hero is always one of the most interesting characters in the MCU films and he really became a selfless hero. However, he has made a lot of mistakes as well.

From creating the murderous Ultron to imprisoning some of his fellow Avengers to nearly killing Captain America and Bucky, Tony can be out of control. It's times like this that fans have a bit of a hard time siding with him. Luckily, he is usually a pretty heroic guy, but he does push his luck at times.

9 Beloved Villain: Mysterio

Mysterio was initially presented as a new mentor to Peter Parker and a hero that could potentially fill the void left by Tony Stark. He was powerful, dashing and noble. However, he soon revealed himself to be a megalomaniac who created a fictional hero to sell his own lies to the world. That's when he really got interesting.

RELATED: Far From Perfect: 5 Things Spider-Man: Far From Home Gets Right About Mysterio (& 5 They Got Wrong)

The idea of a villain creating a hero from scratch to fool the world was a pretty fun idea. Thanks largely to Jake Gyllenhaal's performance, Mysterio was a funny and creative villain who was a blast to watch, even when he was trying to kill everybody.

8 Hated Hero: Hulk

Given Hulk's unpredictable and destructive manner, he can sometimes be hard to love. He certainly has his great moments in which he shows why he belongs on the Avengers, but he's one of the more inconsistent MCU heroes.

In his first film, The Incredible Hulk, he just wasn't a very fun hero to root for. He was just a rage monster who could maybe aim his rampage in the right direction. Though he became a more likable monster later, his refusal to show up and fight in Avengers: Infinity War certainly lost him some fans.

7 Beloved Villain: Killmonger

Killmonger talks to T'Challa while facing death

Sometimes, there are villains you can sort of see yourself agreeing with. Erik Killmonger in Black Panther was a villain that was actually hard to root against. The son of N'Jobu and nephew of King T'Chaka, Killmonger was left orphaned and abandoned in America in order to protect Wakanda's secret.

The movie uses Killmonger's revenge to explore difficult questions, as he sees Wakanda's history of secrecy as the nation turning its back on the struggles of their people. Though his extremist plan was stopped, Killmonger actually succeeded in convincing Wakanda it was time to come out of the shadows.

6 Hated Hero: Quicksilver

Quicksilver had the shortest reign as an Avenger out of all the heroes. He was introduced as a villain who hates the Avengers and sided with Ultron to destroy them. Realizing the error of his ways, Quicksilver and his sister Scarlet Witch joined the Avengers to stop Ultron from destroying the world.

RELATED: The 5 Most Heroic Things Quicksilver Has Ever Done (And The 5 Most Villainous)

Quicksilver is not so much hated as he is met with total indifference by fans. Maybe it's his limited screen time or the fact that the character appeared in a more memorable way in the X-Men series, but even Quicksilver's death didn't move fans as much as it could have.

5 Beloved Villain: The Winter Soldier

Winter Soldier holding the Captain America shield

Though there are plenty of touching friendships in the MCU, one of the most beloved is that between Steve Rogers and Bucky Barnes. These childhood friends had each other's backs before they were super soldiers. That made it all the more tragic when Bucky returned as the brainwashed Hydra assassin, Winter Soldier.

Winter Soldier killed a lot of people, including Tony's parents, but fans knew it wasn't his fault. They only thought of him as Bucky and waited for the moment he returned to his old self.

4 Hated Hero: Hawkeye

Hawkeye Avengers

In a team that has gods and super soldiers in its ranks, a guy who is good with a bow and arrow is bound to be a let-down. Hawkeye has always been a bit of a joke in the MCU. Despite his cocky attitude, he just doesn't seem to add all that much to the team.

Hawkeye was given a family in an attempt to make him more relatable and even poked fun at his own uselessness. However, plenty of fans probably have a different idea of who should have jumped off that cliff on Vormir.

3 Beloved Villain: Thanos

Josh Brolin as Thanos

It's strange to think that a villain who successfully killed half the universe actually has a lot of fans. Thanos was a villain who was teased in the MCU for years, and when he finally showed up to high expectations, he did not disappoint. Not only did Thanos prove to be a formidable foe, but his evil plan was also actually kind of noble.

RELATED: 10 Of The Most Heroic Things Thanos Has Ever Done

Like all good villains, Thanos is the hero of his own story. He was trying to save the universe in his own twisted way. Even after he snapped Spider-Man out of existence, there were some fans who thought Thanos was in the right.

2 Hated Hero: Star-Lord

Star-Lord confronts Thanos about Gamora on Titan

Peter Quill (aka Star-Lord) is certainly one of the funniest and most entertaining heroes of the MCU, but as an actual hero, he's a bit of a screw-up. Quill is still a very immature guy and that can often lead to some pretty stupid decisions, one of which had heavy costs.

As the Avengers try to remove the Infinity Gauntlet from Thanos' hand, Quill becomes enraged on learning of Gamora's death. He attacks Thanos, leading to the Mad Titan freeing himself and eventually fulfilling his master plan.

1 Beloved Villain: Loki

In the early days of the MCU, many critics pointed to the fact that none of the franchise's villains were very interesting. However, one of its earliest baddies is perhaps still the MCU's best. Loki is the adopted brother of Thor who seeks to gain the power he thinks he deserves.

Loki was an incredibly compelling villain in the MCU and even became the first villain to face off against the Avengers as a team. His sad backstory, an interesting arc and a great performance by Tom Hiddleston make Loki one of the most beloved villains around.

NEXT: God Of Mischief: 5 Most Heroic Things Loki Has Ever Done (& 5 Most Foolish)