Here are some examples:
In Comics:
* Thanos: In the "Infinity Gauntlet" storyline, Thanos wipes out half of all life in the universe, including many Avengers. However, not all Avengers are killed in this event.
* The Celestials: In "The Celestial Madonna" storyline, a group of Celestials known as the Exterminators decimate the Earth and kill many Avengers.
* The Kang Dynasty: Kang the Conqueror has repeatedly clashed with the Avengers, causing them significant losses and even seemingly killing some.
In the MCU:
* Thanos: In "Avengers: Infinity War", Thanos successfully snaps his fingers using the Infinity Gauntlet, erasing half of all life in the universe, including several Avengers. This event is reversed in "Avengers: Endgame".
* Ultron: In "Avengers: Age of Ultron", Ultron aims to destroy humanity and nearly succeeds, but the Avengers ultimately defeat him. However, the battle leaves a heavy toll, and some Avengers are left wounded or emotionally scarred.
* The Blip: While not an individual character, the Blip, the result of Thanos' snap, also leads to the disappearance of half of all life in the universe, including several Avengers.
Important Note: In many cases, the deaths of Avengers are temporary, often reversed through time travel, resurrection, or other means. This is a common trope in superhero comics and movies.
Therefore, while various villains have caused the deaths of numerous Avengers, there is no single character who can be credited with killing all of them at once.