The MCU is full of heroic characters whose mission is to make sure that nobody destroys the world or hurts the people the heroes have sworn to protect. Even the best intentions aren't always enough, though, and the heroes don't save everyone. Sometimes, they face characters who are so ruthless that the heroes find themselves at their wit's end, or they're simply overpowered.
The most ruthless MCU characters aren't merciless only when it comes to their enemies but also sometimes those who stand on their side. That makes it even more interesting to watch out for what they do, and it also turns them into a force to be reckoned with.
5 Dreykov
Black Widow wasn't as profitable as other MCU movies, but it had its strong points. One of them was Ray Winstone's Dreykov. Even though he didn't get as much space as other MCU villains, he still comes across as one of the most ruthless characters in the MCU. Not everybody would be willing to transform their daughter into a mindless killing machine, yet Dreykov does it with ease.
What's more, he sees no harm in taking young girls and making them into his servants who would kill for him and die for him as well. Dreykov might not have killed that many people as other villains, but his lack of respect for human lives and their individuality mean he has no redeeming qualities.
4 Ultron
Some MCU villains plan to destroy only their enemies, and they have no desire to kill other people. That's not the case with Ultron, who wants to kill every single living person on Earth because he decides the planet would be much better without humanity. As artificial intelligence went wrong, Ultron has no understanding of morality. He demonstrates this when his army attacks Sokovia, killing hundreds, if not thousands of people.
Ultron once again proves he's ruthless in the animated series What If...? when he's ready to destroy the multiverse and every universe in it. Ambition and a complete lack of respect for human lives, those two things best describe Ultron's attitude toward the world. That makes it even more ironic that Tony Stark originally planned to create Ultron to protect the world, but his creation turned out to be determined to destroy it instead.
3 Red Skull
Red Skull is one of the MCU villains who got what was coming to him after being sent to serve as the Guardian of the Soul Stone, unable to harm anyone again. During World War Two, Red Skull experimented on captured people, including Bucky Barnes. He was a Nazi and presumably killed thousands of people with the help of his troops, and members of Hydra. Red Skull's legacy continued to impact the MCU long after his disappearance as the TV series Agents of S.H.I.E.L.D. shows.
Red Skull's legacy is full of hate toward those he considered less worthy of living, betrayal, and murder. Number-wise, he's one of the villains who did the biggest damage in the MCU, and until his last moment on Earth, Red Skull showed no signs of regretting his actions, which separates him from the likes of Loki or Ghost, to name just two.
2 Thanos
Red Skull's actions might have led to the death of thousands of people, but Thanos was even more successful in his evil plan. Thanos had the goal to erase half of all life in the universe, and he succeeded. That's not counting the many other people he killed, such as Loki and Vision. The superheroes reversed the snap, but the damage was already done by then. Thanos had moments when he showed genuine emotions, such as his remorse over throwing Gamora to her death to gain the Soul Stone.
But even the death of his daughter at his hands wasn't enough to stop Thanos from executing his plan, showing that he had no time for remorse and was ready to be ruthless. Thanos is pragmatic in his actions. He mostly approaches them calmly unlike more emotional villains, and that makes him even more terrifying since he doesn't stop until he fulfills his plan.
1 Ego
Not only does Ego have an ego the size of a planet, but he's even worse than Thanos when it comes to killing his children, people who should be the closest to him. As Peter Quill eventually learns in the second Guardians of the Galaxy movie, Ego has fathered a lot of children during his long life but not because he wanted a big loving family but because he wished to use their potential powers for himself.
When Ego learned that Peter had inherited his powers and realized that his son wouldn't do what he wanted him to do, he tried to kill Peter. Ego's ruthlessness toward his own children marks him as one of the evilest MCU characters. Fortunately, he made the fateful mistake of telling Peter he killed his mom, which enraged Peter enough to defeat his father once and for all.