In season 4 of Supernatural, the passionate fandom was introduced to Castiel, a special type of angel who had an epic introduction into the show and quickly became one of the best, most beloved parts of the show. Most of the time, Castiel had the best of intentions, trying to help the Winchesters, help heaven, and help humankind, as he was a hero like the other members of Team Free Will.
But, it was never that simple for Cas, just like it was never so black and white for Sam and Dean. Castiel made many mistakes, and alongside his numerous examples of heroics, he proved himself to be villainous at different times in the show.
Hero - Trying To Bring Heaven Together
When the angels were cast from Heaven by Metatron, so many of them were lost; they needed leadership, someone to follow, a cause to believe in, and Castiel offered it to them.
Whereas other angels had tried to capitalize on what had happened, and Metatron was trying to retain his God-adjacent status, Castiel tried to rally the angels together to take back Heaven, defeat Metatron, to take them back home. It was a rocky road, and even though they all left him at one point, Castiel did help bring normality back for the angels.
Villain - Killing Jane
Before he did that, though, Castiel unknowingly helped Metatron cast the angels from Heaven by helping him perform a ritual he believed would be closing Heaven's gates.
There were three steps to the ritual, one of which was pretty brutal, cutting out the heart of and killing a Nephilim. Metatron took Castiel to Jane, a Nephilim living as a waitress. Quite simply, Castiel killed her, and one of the two cut out her heart, an ingredient of the ritual.
Hero - Protecting Kelly & Jack Kline
It is definitely odd to think about the fact he killed a Nephilim only a couple of years before he pretty much fathered one in Jack Kline. Despite a lot of initial hesitation and fear, Castiel saw how good Jack could be and decided to help Jack and Kelly.
Castiel kept Kelly on the move, hiding her, protecting her, and making sure Jack would be born healthy, not being hunted. Castiel saw the good in Jack before he was even born and decided to save his life along with Kelly's because of it.
Villain - Stealing Theo's Grace
Another ingredient in the ritual to expel angels from Heaven was the grace of an angel, which Metatron obviously took from Castiel, leaving him powerless and lost when he arrived back on Earth.
Eventually, Castiel got grace by slitting the throat of fellow angel Theo (who had tortured Castiel admittedly) and stealing his grace before smiting him. This may not seem like the most villainous thing to fans given the circumstances, but to angels, to Castiel and others, it was seen as a crime of the highest order and caused immense pain for whoever's grace was being stolen.
Hero - Betraying The Angels & Helping The Winchesters
While Castiel was quickly beloved by fans, he did spend a lot of his debut season on the side of the angels, who were far from likable and were just another villain on the level of the demons.
Thankfully, though, when Zachariah and the angels trapped Dean to stop him from stopping Sam from killing Lilith and unleashing Lucifer, Castiel banished Zachariah. He joined Dean, helped him save Sam, and through season 5, he was another member of Team Free Will.
Villain - Helping Kidnap & Kill Oskar
The Mark of Cain caused whole heaps of bother not just for Dean, but for the allies of the Winchesters who tried everything to get rid of the Mark, including using the Book of the Damned behind Dean's back.
One step in removing the Mark required Rowena to kill someone she loved, and that's where Oskar comes in. While it was Crowley who retrieved Oskar, Castiel was fine sitting idly by as Rowena stabbed him in the neck and bled him out.
Hero - Sacrificing Himself for Jack
The relationship between Castiel and Jack is pretty great to watch for fans, and Cas undoubtedly holds Jack, who is essentially his adopted son, in such a special place in his heart.
When Jack died of illness in season 14, it was heartbreaking for everyone, but was made even worse by the fact the Shadow was trying to take him from Heaven and claim him for the Empty. That was when Castiel stepped in and struck a deal for Jack's life, which would see the Shadow eventually take Castiel when Castiel had reached true happiness.
Villain - Working With Crowley
Season 6 was pretty much the worst, most unlikable version of Castiel fans had ever gotten, and that was largely down to the fact he was secretly working with Crowley behind the Winchester's backs.
In the war against Raphael, Castiel used souls from Hell given to him by Crowley in exchange for his help in opening Purgatory. One of the most heartbreaking aspects of this storyline is that despite Bobby and Sam's suspicions, Dean stayed as loyal and faithful to Castiel as he could for so long, not believing he could work with Crowley behind their back.
Hero - Sacrificing Himself For Dean
As mentioned earlier, Castiel made a deal with the Shadow that he would be taken to the Empty when he had found true happiness. In "Despair," that happiness finally got revealed; Dean Winchester.
Looking like they had no escape from Billie, Castiel realizes that the Shadow could take Billie and so summons the entity. He then proceeds to heartbreakingly confess his adoration, his respect, and his love for Dean, telling Dean to flat out that he loves him in one of the most emotional Supernatural scenes ever. This triggers Castiel's true happiness, and he is taken by the Shadow to the Empty alongside Billie, saving Dean.
Villain - The Entire God/Leviathan Ordeal
Working with Crowley was really not that big of a deal compared to what he did when he deceived Crowley and opened up Purgatory himself.
When Castiel opened up Purgatory, he absorbed all of the souls from there, becoming an omnipotent being. He deemed himself God and gave the Winchester's mercy, although he warned them he would kill them if they oppose him. He then kills thousands of angels, killing a load of humans too, eventually succumbing to the Leviathan. Castiel thought he was doing good like he so often does with his mistakes, but he was well in over his head.