The Winchester brothers of Supernatural spent fifteen seasons saving the world from various manner of being, entity, diety, and creature, ranging from a Wendigo to Princes of Hell to God himself. As individuals, they are just as heroic, with Sam doing so many things on his own that are brave, world-saving, and selfless numerous times throughout every season of the show.
Alongside these heroics, though, there are many times when Sam finds himself making mistakes, doing things that are far more villainous than they are heroic. These are usually either due to him being manipulated or because he thought he was doing good.
Hero - Letting Castiel Extract His Grace
In season 9, after Gadreel left Sam's body, Sam and Castiel find out that when an angel leaves their vessel, a small part of them is left behind, known as their grace.
To try and locate Gadreel, Sam insists Castiel extracts the grace, even though it is reverting his body back to its state pre-Gadreel. This may be seen as stupid, and thankfully Castiel healed Sam, but Sam was still willing to do anything that needed to be done for the sake of Kevin and taking out Gadreel and Metatron.
Villain - Killing Emma
Dean Winchester never really got to have a family of his own. He got close with Lisa and Ben, a brief reunion with his father and mother in Lebanon, and even in the bunker with Sam, Cas, and Jack, but he never officially got it.
He did, though, have a daughter, Emma. Emma admittedly did plan on killing Dean, but Dean was so ready to give her a chance. Innocent until proven guilty, he wanted her to live a life and not be brainwashed, and Sam killed his niece with relative ease before that could happen.
Hero - Saving Hostages From The Warehouse
The raid on Niveus Pharmaceuticals saw Sam, Bobby, and Cas go to the demon warehouse where Pestilence was planning to distribute the Croatoan virus.
There, Sam was nothing short of a hero. Many humans were trapped there, and he saved all of them, something which Bobby comments on to Dean; it was incredible how selfless and how ready to help everyone Sam really was, taking out demon minions of Pestilence in the process.
Villain - Lying To Dean
Characters returning from deaths, angel-on-angel battles, heartbreak, and the Winchester brothers lying to each other are some of the constants in Supernatural, with Sam and Dean guilty of the latter throughout most seasons of the show.
Sam's lying to Dean, like Dean's to Sam, seemed so awful and unnecessary a lot of the time. That included lying about Ruby, demon blood, the Book of the Damned, not letting him know he had returned from Hell, and the list goes on and on. A lot of it can be inoffensive but still does not paint either brother in the most heroic light when they do it.
Hero - Taking On The Trials
The trials that would result in the closing of the gates of hell forever, preventing demons from ever again walking the Earth, were supposed to be undergone by Dean. However, Sam getting the kill on the hellhound resulted in him being thrust into the role.
Dean wanted to take them on, but Sam was adamant he was on course and would complete them. They took so much out of him, draining him of all health and life, but he was so prepared, determined, and strong enough to complete them, which he would have done, killing himself, had Dean not stopped him.
Villain - Getting Hooked On Demon Blood
When Dean returned from Hell in season 4, there was clearly something going on with Sam, which he successfully hid for not very long, with it becoming known to fans quickly that he was in cahoots with Ruby.
Worse than that, though, was the fact that Sam was also hooked on demon blood. He may have denied being so, but Sam was addicted. He was easily manipulated by Ruby, slept with her, and entered some undefined relationship that led him down a dark path.
Hero - Curing Dean
Demon Dean, a.k.a Deanmon, was an iconic, short-lived time of Dean's life in Supernatural and came after a rough patch in the brother's relationship.
Despite this rough patch, Sam hated seeing his brother die and hated even more seeing him become a demon. He hunted and went after Dean in a desperate attempt to bring him back into normality. Eventually, he succeeded, risking his life to capture Dean with the help of Cas and cure him like he almost did Crowley.
Villain - Going Through With Killing Lilith
It is understandable why Sam wanted to kill Lilith. But, the fact of the matter is, the fans knew this was an awful idea, and on top of that, the way Sam went about trying to do it was worse.
He literally became a demon by drinking so much of their blood. He sucked dry a conscious human who had a demon inside them and he fought with not only Dean but Bobby and allowed Ruby and his hatred to manipulate him. It resulted in him not simply killing Lilith, but unleashing Lucifer and the Apocolypse on the world.
Hero - Jumping In The Pit
In the same breath, though, it must be said that even though Sam caused Lucifer to be unleashed on Earth and just about brought on the Apocolypse, he stopped it and put Lucifer back in the cage.
"Swan Song" is widely viewed as one of the best Supernatural episodes ever, and it is hard to argue against that with so many incredible moments. Despite Lucifer's power, Dean, the Impala, and memories brought Sam back, gave him control, allowed him to take over his body, and jump in the pit with Lucifer. It saved the world in epic, heartbreaking fashion.
Villain - Soulless Sam
There have been different eras of the Winchester boys, different times of their lives that mattered in the story where they were not their regular selves, like Demon Dean, demon blood-filled Sam, and, of course, soulless Sam.
Soulless Sam was not a hero. He was a hunter and he did good sometimes but he was not Sam, and ergo was not a hero. He was ruthless, cold-blooded, and selfish. Without a soul, Sam tried to kill Bobby; he did not care about Dean and was overall so unlikable, even to the most hardcore Sam fans.