The Avengers is the superteam in Marvel Comics that almost every major hero has been a member of. Much like the Justice League in DC Comics, the Avengers formed when the threat was too large for one person to defeat. And in this case, a Loki-controlled rampaging Hulk. Then, throughout the years, it was the home of a who's-who of Marvel Comics.
Throughout the years, the Avengers have won and lost major battles, and they have suffered great losses along the way. There were some deaths of prominent Avengers members when the team was battling villains, and there were even some occurring when the Avengers battled each other.
Wonder Man
Wonder Man had a long run with the Avengers, working on different iterations of the team, and he was even partially responsible for the creation of Vision. That would not have happened if Simon Williams had not died. He worked as a part of the Masters of Evil but ended up turning good and saving the Avengers in the battle. However, to save the Avengers, Wonder Man sacrificed himself and died in Avengers #9 by Stan Lee and Don Heck. His brainwaves were later used to help create Vision.
Iron Man
One would think the death of Iron Man would rank high when it comes to heartbreaking moments. However, the '90s was not a nice time for Iron Man. His death came during "The Crossing" storyline where Iron Man turned out to be a villain, a mole since he first appeared with the Avengers so many years ago.
He worked for Kang and committed some terrible acts, even almost killing Wasp. In Avengers #395 by Bob Harras, Terry Kavanagh & Mike Deodato Jr., Iron Man sacrificed himself in the end and was replaced by a teenage version of himself from the past.
Scarlet Witch
Scarlet Witch has done several bad things in her life and killed at least two people on this list. However, when she died, she was completely innocent of any wrongdoing, and it was a tragic moment. Captain America set up a team of Avengers and mutants to prove the two sides could coexist.
The problem is that Rogue hated Scarlet Witch for M-Day, and they never got along. In Uncanny Avengers #14 by Rick Remender and Steve McNiven, Wanda was trying to help save the world, and Rogue, thinking she was trying to do something evil, murdered her. Before Wanda died, she completed her spell and admitted her love for Wonder Man.
Ant-Man
The Scarlet Witch went mad in the "Avengers Disassembled" storyline when Wasp accidentally mentioned her twins. Years before, Wanda had created children for her and Vision, but they were pieces of Mephisto, and Agatha Harkness had to destroy them.
While the Avengers didn't know it, they ended up fighting what they thought was a Kree invasion. Before that, in Avengers # 500 by Brian Michael Bendis and David Finch, Jack of Hearts returned from the dead and then exploded again, killing Scott Lang instantly.
Wasp
Wasp was one of the original Avengers, and through the years, she went from a stereotypical flighty girl to a true leader of the team. She almost died when Tony Stark turned evil but was transformed into a bug-like creature. She then returned to normal later and went on to remain a core Avengers member.
While helping defend the planet in Secret Invasion #8 by Brian Michael Bendis and Leinil Francis Yu, Wasp died. She used the Pym growth serum but then was turned into a chemical bomb, forcing Thor to use his power to destroy her to save everyone.
Mockingbird
Hawkeye and Mockingbird had a whirlwind relationship, where they two met, fell in love quickly, and married with little dating or getting to know each other. Eventually, they went together to form the West Coast Avengers, but their marriage slowly fell apart. It didn't matter to Mockingbird, though, as in Avengers: West Coast #100 by Roy Thomas and Don Hudson. She was there to sacrifice herself to save Hawkeye in a battle with Mephisto. It messed Hawkeye up so badly that he tried to go to Hell to save her.
Hawkeye
When Scarlet Witch turned evil after remembering that she had children taken from her, she single-handily destroyed the Avengers. She started by killing Agatha Harkness, but that was not discovered until all was said and done. She also killed Ant-Man.
The biggest death here was Hawkeye. In Avengers # 502 by Brian Michael Bendis and David Finch, Scarlet Witch used her powers to make the world think the Kree Empire was attacking. Hawkeye went up to stop them himself and died believing he was saving the Earth. It was one of the biggest deaths in Avengers history.
Black Widow
The Black Widow is a stone-cold assassin, and she has no problem taking anyone out that she needs to. However, she also has a weak spot when it comes to those she trusts above all others. One of those people is Captain America.
In Secret Empire #7 by Nick Spencer, Andrea Sorrentino, Rod Reis & Joshua Cassara, Captain America, who had taken over America in the name of Hydra, was battling Miles Morales and Black Widow showed up to help protect Spider-Man. She paid the price when Cap hit her with his shield, breaking her neck and killing her instantly. It was the ultimate betrayal.
Captain America
The first Civil War crossover series saw the heroes from both sides battling for what they believed in. There were deaths and betrayals along the way, but the most shocking came at the end. Captain America turned himself in and allowed the government to arrest him. However, in Captain America Vol. 5 #25 by Ed Brubaker and Steve Epting, as he was led into a courthouse, a hypnotized Sharon Carter assassinated the Avengers greatest hero. As Iron Man watched over his fellow Avengers teammate, he realized the great cost of the war.
Stature
While she is not the most popular member of the Avengers, Stature's death was heartbreaking for a very different reason. She was a teenager, and she was part of the new Young Avengers team. The Young Avengers had set out to find Speed and Wiccan, who themselves set out to find their mother, the Scarlet Witch.
Stature then saw her greatest dream come true as her father, Scott Lang, returned from the dead. Her dream was short-lived as the team battled Doctor Doom and the villain killed Stature, the teenage Avenger dying in her father's arms.