There are various ways to define someone as powerful. Some people equate power with a person's strength or status or influence over others. Ultimately, it comes down to a person's ability to reduce their opponents to nothing. In the Marvel Universe, some characters can literally reduce opponents to nothing.
Amidst the multitude of mutants, Inhumans, vigilantes, and aliens, there exists a select few with devastating powers. These people can end a fight in seconds just by making direct contact with someone. These are the people whose touch is lethal.
10 Rogue
Anyone who knows about the X-Men should be familiar with Rogue. Her mutation allows her to sap away the powers, memories, abilities, energies, and life from anyone she comes in contact with. She has yet to kill anyone in the main continuity but she's gotten close several times and put her first crush in a coma.
As a result, Rogue has often placed herself at a distance from other people fearing that she might harm them unintentionally. Thank goodness she found a new family in the X-Men.
9 Elixir
Classified as an Omega-level mutant, Joshua Foley, aka Elixir, was born with the dichotomous ability of biokinesis. That means he can create biomolecular energy that can affect organic matter depending on his intention.
If he desired, Elixir could heal people suffering from fatal injuries and regenerate damaged organs. On the flip side, he can also cause harm to the body and kill people, as he did with William Stryker. Elixir was last seen living on Krakoa as part of the Five but time will tell if he will remain as a healer.
8 Deadly Ernest
Deadly Ernest was one of Alpha Flight's deadliest villains. Ernest was a WWI soldier who almost died on the battlefield from mustard gas. When Death came for him, he somehow rejected it. As a result, Ernest was cursed with immortality and the ability to instantly kill any he touches.
Ernest learned this lesson the hard way when he accidentally killed his wife by hugging her. He came a notorious criminal until he provoked Northstar and Nemesis. Ernest's body has been destroyed and sliced on two different occasions and he has yet to make a third appearance. In the realm of comics, it wouldn't be hard for a writer to resurrect him.
7 Wither
Kevin Ford is another one of the many mutants who got dealt a bad hand in life. Aptly named Wither, his powers causes any organic matter he touches to disintegrate and decay. Unlike Rogue, his powers act faster and if they don't kill someone they will definitely cripple them.
After accidentally using his powers on his crush's arm, Wither ran away and wound up joining Selene Gallio. He killed several police officers and his classmate Onyxx before Elixir stopped him. He was presumably resurrected by the Five on Krakoa but it's unclear if he is repentant.
6 Mortis/ Lois London
Dazzler is arguably one of the most powerful mutants in the X-Men but she's not as well-known as other characters like Wolverine or Magneto. If she was then maybe readers would be familiar with her half-sister Lois London, aka Mortis. The two share the same mother but their powers couldn't be any more different.
Mortis's body can produce highly destructive energy capable of forcing life energy out of people from a distance or by grazing them. Since Dazzler seems to be the only person immune from her half-sister's touch, these two are fated to fight each other.
5 Shiklah
It might come as a surprise to many that a few years ago Deadpool got hitched and married Queen Shiklah, ruler of Monster Metropolis. Actually, that sounds pretty on-par with Deadpool's taste in women. As a succubus, Shiklah can suck the life force out of anyone through sexual acts.
Part of the reason they worked well together was that Shiklah could kiss Deadpool without killing him. Too bad their happy little marriage didn't last. Now both of them are going to have trouble finding someone who can survive a night with them.
4 Carrion
The Clone Saga introduced a multitude of new characters and clones into the Marvel Universe along with the Carrion. Three different people held the mantle of Carrion, the first being a failed clone of the Jackal and the other two victims of the Carrion Virus. They were all practically identical in appearance, abilities, and their hatred of the web-slinger.
One of the powers each version possessed was a decaying touch that could turn organic matter into ash. It's so potent that when Malcolm McBride nearly died when he turned the decaying touch on himself. He would have perished if not for Shriek performing a rare act of kindness.
3 Hela
Thanks to Thor: Ragnorak and Kate Winslet, Hela has become a more prevalent and recognizable figure in recent years. The movie took several liberties with her character and didn't even begin to address how fatal she can be. As the goddess of death, Hela's touch is just as lethal to mortals as it is to Asgardians.
Without Hela, Asgardian souls would remain trapped in their bodies even after death and cannot reach the afterlife unless she guides them. What's troubling for her is that there are some stubborn souls like Thor who won't go peacefully into that good night.
2 Bloodscream
Whenever someone on this list touches someone, the victims' suffering ends once they perish. For Bloodscream's victims, death would be a courtesy. When Bloodscream sucks the life force out of his victims, they shrivel up and are reduced to a mummy-like state.
While his victims might survive being drained, albeit, with a scarring red handprint, those who die have the misfortune of potentially becoming zombie-esque beings under his control. Talk about a fate worse than death.
1 Magnus Lehnsherr
The multiverse is full of alternate realities serving as homes to characters that wouldn't normally exist in the mainstream continuity. Such is the case with Magnus Lehnsherr, the son of Rogue and Magneto.
As to be expected of an offspring of two mutants, Magnus's mutation is a combination of his parents. What's special about him is that when people die from his touch, they don't just collapse to the floor in a crumpled heap. Whenever Magnus touches anyone, they are turned to solid steel. Not exactly a Midas touch but pretty close to it.