The X-Men have had more deaths than just about any superteam in history. It has actually reached the point where most people are not surprised when someone dies, and the deaths never mean anything anymore. In today's Marvel Universe, X-Men die all the time and have a resurrection protocol to bring them back.
However, in the early years of X-Men comics, there were deaths that shocked readers and left the team heartbroken. There were also some deaths in more recent years that were shocking enough to stun fans and leave them mourning the loss of a mutant favorite.
Cypher
The idea of Cypher dying in New Mutants #60 by Louise Simonson and Bret Blevins was not something that broke the readers' hearts. The truth is that Douglas Ramsey was poorly written and built.
No one cared about him, and it was the equivalent of Jason Todd dying in Batman comics. However, this was a death that devastated his teammates, with Dani Moonstar and Illyana Rasputin heartbroken and Warlock shattered.
Cyclops
What lessens the impact of Cyclops' death in the Inhumans vs. X-Men event series is that he was a martyr for too long and had proven to be a less than heroic character.
There was an event where a cloud of Terrigen Mists spread over the Earth, which was fatal to mutants. Cyclops fell quickly to the Terrigen Mists and died. However, what made this even more heartbreaking was that Emma Frost hid his death and created an illusion of a still-alive Cyclops, who Black Bolt "killed."
Wolfsbane
Out of all the New Mutants , Rahne Sinclair was the one with the most inner trauma. That made her death not only heartbreaking but truly infuriating when looking at the rampant racism in the world toward mutants. The death was shown in Uncanny X-Men Vol. 5 #17 by Matthew Rosenberg and Carlos Gomez.
Wolfsbane had quit the X-Men and was sitting alone in a park. When some guys came up and started to hit on her, she asked to be left alone. They started to abuse her physically, and when she showed she was a mutant, they beat her to death. Wolverine skipped her funeral to get revenge.
Kate Pryde
In the days of Krakoa, no mutant stays dead. When a mutant dies, The Five can bring them back from the dead. This is the resurrection protocol and only failed to work when someone died in Otherworld. There was also one death that almost didn't get resurrected.
Sebastian Shaw murdered Kate Pryde and threw her into the sea. As long as she was not found, she couldn't be resurrected. However, the Marauders found her, but she still couldn't be resurrected due to her phasing powers. It was heartbreaking, as Pryde had never died before. The Five finally found a way to bring her back.
Professor X
Professor X had died more than once. There were also times where Charles Xavier faked his own death for one reason or another.
However, in Avengers Vs. X-Men, the Phoenix Force returned to Earth, and when the Avengers tried to stop it from getting to Hope Summers, it ended up creating the Phoenix Five. This led to Professor X trying to stop them, and Cyclops murdered him, although it was while under the Phoenix's influence. This turned Cyclops into a fugitive and a martyr to mutants everywhere.
Wolverine
It was thought that Wolverine would live forever and was pretty much immune to death. That was thrown out the window when he was infected with a Microverse virus that killed his healing factor. However, this caused him to become dangerously vulnerable in fights.
It came to an end in Death of Wolverine #4 by Charles Soule and Steve McNiven. While saving test subjects in a new Weapon X program, Wolverine had liquid adamantium cover his body and harden, suffocating and killing him.
Nightcrawler
When it comes to the of X-Men, Nightcrawler has been one of the most beloved in history. He had always suffered from self-doubt and overcame this time and time again. Nightcrawler died while doing what he does best — protecting others.
In X-Force Vol. 3 #26 by Craig Kyle, Christopher Yost & Mike Choi, Nightcrawler was on the run with Hope Summers, trying to escape from the murderous Bastion. Because this villain was a master at killing mutants, it predicted a Nightcrawler port and drove his fist through Kurt's chest. As a true hero, Nightcrawler lived long enough to port Hope to Utopia and safety.
Magik
Illyana Rasputin was a child mutant, the younger sister of Colossus, but she went through a tough life when she was transported to Limbo and left there for years. She ended up de-aging in Inferno, but she ended up infected with the Legacy Virus.
Her death in Uncanny X-Men #303 by Scott Lobdell and Richard Bennett was one of the most heartbreaking moments in comic book history. Jubilee, who refused to care about anyone, found herself sucked into the little girl's fate, and Kitty Pryde was there to say goodbye to her best friend when she ended up the first to die of the virus.
Jean Grey
Jean Grey's deaths in comics have become a running joke to the point where no one cared anymore whether she lived or died. It was just ridiculous how many times the mutant died and returned. However, the first time that she died, it was shocking and heartbreaking.
The Phoenix Force had consumed her, and the Hellfire Club had broken her. She slaughtered an entire galaxy and was sentenced to death. In X-Men #137, the X-Men battled the Shi'ar Empire for her life, but Phoenix sacrificed herself to save her teammates, breaking Cyclops completely.
Colossus
Out of all the X-Men in history, the one with the kindest heart and deepest soul was Piotr Rasputin, the mutant known as Colossus. During the Legacy Virus era, a disease that already killed his little sister Illyana, Beast believed he found a cure. However, it required a sacrifice.
One mutant would take the cure and activate his powers to spread it to mutants worldwide. Colossus agreed to do it. When he activated his powers, it worked and saved all infected mutants across the planet but killed Colossus immediately in one of the greatest sacrifices in X-Men history.
