Of all the heroes in the Marvel Cinematic Universe, it can be difficult to determine which one has killed the most. Obviously, the highest kill count among all the characters in the MCU is Thanos, as he managed to use the Infinity Stones to eradicate half of the universe. However, while many villains have high kill counts, be it Guardians of the Galaxy's Ronan the Accuser or Ego the Living Planet, there are some heroes with pretty high kill counts of their own.
Some Avengers have taken lives on-screen and some have killed off-screen, leaving heavy tolls in their wakes. Taking these factors into account, the identity of the Avenger with the most blood on their hands is actually quite surprising.
Who Has The Most On-Screen Kills?
While the Avengers do have several on-screen kills during their various fights, there are a few characters with a surprisingly small number of kills. Many characters who appeared in Phase 3, such as Doctor Strange, Spider-Man and Black Panther, have only taken life a few times and only when absolutely necessary. Vision and Ant-Man have likewise taken few lives, with Ant-Man's most significant kill being Yellowjacket. Wanda inadvertently killed people in Lagos during Captain America: Civil War and killed some of Thanos's forces in Avengers: Infinity War, but not many beyond these instances. Carol Danvers is the only exception here, but it's hard to determine just how many people she killed while serving the Kree or afterward.
Several military figures and assassins in the MCU have surprisingly few on-screen kills. Black Widow and Hawkeye, despite being trained to kill, have ended few lives, though presumably their off-screen violence has left its mark, especially Hawkeye's days as Ronin during Avengers: Endgame. The same is true for Captain America, who despite fighting in World War II, one of the bloodiest wars ever fought, has killed few people during his lifetime. This is in part because he often avoided killing, even when locked in mortal combat. Unexpectedly, the Hulk's total kill count is also rather low on-screen, though he presumably killed hundreds off-screen, particularly during his time on Sakaar.
When it comes to on-screen kills, however, Tony Stark leads the charge by a significant amount. Within his first film, he kills dozens of members of the Ten Rings in his pursuit to clear them out. This is followed by The Avengers, where he nukes a city-sized Chitauri ship, killing all the soldiers already deployed on Earth. Iron Man 3 and Avengers: Age of Ultron continues this with even more bloodshed, though it's debatable if Ultron's kills are directly Tony's fault. Finally, it's undeniable that when Stark takes the Infinity Stones in Avengers: Endgame, the obliteration of Thanos's army is completely on him.
Tony Stark vs. Thor: Who Killed More?
Of course, omitted from the prior discussion is Thor Odinson, who killed a significant number of adversaries over the course of his films, albeit fewer than Stark. However, Thor has had a much longer life than Stark. This means that he had far more time to build up his kill count. As he mentions in Avengers: Infinity War, he's "fifteen hundred years old. I’ve killed twice as many enemies as that." Taken literally, this puts Thor's kill count at over three thousand, but it's more of a rough estimate.
As a warrior with far more time, there's no denying that Thor has killed multitudes of people in war -- so many he might not be able to keep track. So ultimately, the question becomes: did Thor, in his over a thousand years of life, kill more people than Tony Stark, who has killed thousands upon thousands on-screen?
Ultimately, even given Thor's estimate of three thousand, Tony Stark might've killed more people on and off-screen, due to not only his massacre of the Chitauri and Thanos's army but also the sheer amount of damage caused by the weapons he created. This isn't even accounting for Ultron, but just his company's missiles give him a leg-up. As shown in WandaVision, Stark's weapons caused untold damage all over the world for many years. Even HYDRA, through S.H.I.E.L.D., used Stark's weapons, as seen in Captain America: The Winter Soldier. Tony Stark has outdone Thor in just a couple decades, which is disturbing for countless reasons.