The Mark of Cain impacted Dean throughout two seasons of the show. It controlled Dean and made him do uncharacteristically bad things.
Throughout Supernatural, both Sam and Dean Winchester find themselves possessed or controlled by different beings and phenomenons. Both get possessed by angels, Sam drinks demon blood, and Dean wields the Mark of Cain, which takes over Dean at an increasing rate over two seasons.
The Mark, inherited by Dean from its namesake, led Dean’s personality and temperament to change on multiple occasions, unleashing its violent tendencies and making Dean do uncharacteristically bad things.
Above & Beyond Aggression
This is not necessarily one bad thing Dean does, but a slew of questionable acts that highlight how the Mark was changing Dean and how that was not good.
He repeatedly stabbed at Abbadon’s dead body, he violently killed Cuthbert, he was easily irritated and found himself frustrated at both Sam and Castiel, and throughout his time with the Mark held a bloodlust, as well as a desire for the First Blade that made him, at times, unlikeable.
Murdering Randy & Thugs
In “The Things We Left Behind,” Sam and Dean help Castiel reconnect with Jimmy Novak’s daughter Claire who has been taken in by Randy. Randy uses Claire to steal and pay off his debts.
When Randy is found by thugs collecting said debts, he betrays Claire, offering her up to the loan sharks. Fast forward to after Team Free Will saving Claire–Dean gets provoked by the men inside, and Sam, Cas, and Claire arrive back inside to see Dean covered in blood, surrounded by bodies, having slaughtered the men mercilessly thanks to the Mark.
The Mark of Cain so often makes Dean’s anger, bloodlust, and general violence almost uncontrollable. One of the prime examples of these effects is when he almost kills Metatron.
After Cas breaks him out of angel prison, Dean interrogates the Scribe of God for information but is so easily provoked by Metatron that he nearly kills him. While Dean would never usually fall for petty provocations from the likes of Metatron, the Mark of Cain forces his hand.
Costing Castiel His Support
When Metatron was the main villain and an actual threat, Dean was interrogating someone who turned out to be a double-agent of Metatron’s in the form of Tessa, an old sort of friend of Dean’s.
Dean took the First Blade with him to Cas’s base of operations and into his interrogation with Tessa, lying to Sam about doing so, and accidentally killing Tessa.
When Cas refuses to kill Dean, his angel army abandons him in the fight against Metatron. Dean’s inability to separate himself from the First Blade and the fact he pulled it out to threaten Tessa were moves made by the Mark’s influence and almost cost them the fight against Metatron.
That was, of course, not the only time Dean nearly cost the heroes in their fight against Metatron. Later in the very same episode the Mark properly takes over Dean.
When Gadreel arrives at the bunker to offer his support in the fight against Metatron, Dean lashes out and cuts him with the First Blade, almost killing the angel who was genuinely trying to help the heroes. Dean would have slaughtered him if he were not held back.
When Charlie Bradbury made her return from Oz in season 11, it was, as usual, a welcome sight for fans and a unique appearance from the character as she was split in two, her good side and her dark side.
Dean finds himself at blows with Dark Charlie later in the episode and, again, loses his cool while fighting her, forgetting that harming her also causes damage to Charlie.
Nearly Killing Castiel
Fast forward to the end of that same season, and Dean has pretty much lost it and is both desperate for the Mark of Cain to be gone but also still feeling its pull and its effects.
At the end of “The Prisoner,” Castiel confronts Dean, and Dean gets angry with Cas because he lied to him about using the Book of the Damned to get rid of the Mark. Dean overpowers and beats Castiel, almost killing him with the angel blade, leaving him bloody in the Bunker.
Killing Cyrus Styne
But why did Castiel confront a volatile Dean? That was because Dean murdered, in cold blood, Cyrus Styne, who was with his family as they tried to tear apart the Bunker.
Dean escaped the grip of the Styne family and murdered them all at their home, found his way to the Bunker, and killed the rest, with Cyrus, who hated his family and what they did, begging for his life. Dean shot him in the head in cold blood, not listening to his pleas.
Releasing The Darkness
Unlike the above entries, this was not something Dean did under the influence of the Mark of Cain or because of its effects on him, but rather a by-product of him having the Mark in the first place.
The Book of the Damned, which got rid of the Mark of Cain, did not just do that but also brought about the Darkness. Dean could not kill Sam; he chose to kill Death and release the Darkness to save his brother and remove the Mark of Cain.
Becoming A Knight Of Hell
This is also a bit different from other entries; it was not something Dean deliberately did because of the Mark of Cain, but it was the Mark of Cain’s doing, and without it, Deanmon would never have happened.
Dean’s desperation to murder Metatron and his attachment to the First Blade led him to the angel who killed him. As Crowley knew, the Mark had to keep Dean alive, and so he became a demon, or rather, a Knight of Hell, doing a lot of evil during his time as such.