Castiel started out on Supernatural as an obedient character. When he did show critical thinking skills, it got him into trouble with his superiors. Turns out, he was following orders from some not-so-benevolent beings. But despite making the moral choice to support humanity, he wasn't really prepared for the messiness of making his own decisions when he sided with the Winchesters.
That's bad news considering that heroism is a risky business. Castiel does the best he can, and sometimes the risks have panned out for him. Other times, they've gone catastrophically wrong. Here are some of the riskiest things, both successful and not, that Castiel has done.
Helping Sam & Dean Time Travel
Every science fiction fan can spot the problem with time travel. The potential for nonsense time loops and accidentally ruining the world with the smallest of actions is terrifying. Castiel helped the boys time travel not just once, but twice.
Once he didn't have much of a choice since Anna had already gone back in time to stop Sam's birth. The second time, however, wasn't cleaned up so neatly. He sent the brothers to get Phoenix ash, and nearly got them stuck because he was too low on power to bring them back. He had to juice himself up by touching Bobby's soul. And the Phoenix ash had to be delivered by US mail courtesy of Samuel Colt.
Teleporting Dean to Sam
At the moment of truth when Sam was about to kill Lilith, Dean was desperate to talk to him. He didn't know what was going on while he was in lockdown, he just knew something bad was happening. Castiel, on the other hand, knew exactly what was going to happen. And he wasn't supposed to say a peep.
Even though Dean begged to see his brother, Castiel kept telling him no. Finally, Dean got through to him and Castiel made the critical choice to betray his superiors. He blasted Zachariah out of the room and teleported Dean to his brother. Castiel lost his life (for the first time) and Dean failed to stop Sam. He was just too late.
Killing Billie
After another ill-advised supernatural deal by the Winchesters, Mary Winchester was left holding the bag and about to commit suicide to satisfy the reaper who wanted one of her sons. Castiel took the novel approach of "screw that" and just stabbed her instead.
Cheating on a deal with death is obviously a bad idea. Castiel just didn't know the specific consequences would be Billie resurrecting as the new Death, head of all the other reapers. It seems to have worked out okay, but it's hard to forget that Billie is probably itching to reap him.
Swallowing Monster Souls
One of Castiel's most infamously bad decisions was his strategy for winning the civil war in heaven. While his plan technically worked, the fallout was way worse than he imagined.
The single worst part of his plan was swallowing the souls of Purgatory, which included the Leviathans. He didn't realize this would make him explode and release them all into the world, inadvertently causing a different type of apocalypse than the one he was trying to stop. For a risk that Castiel calculated for so long, it's strange that it failed so spectacularly.
Burning Michael
Castiel knew there was no hope left when Michael and Lucifer were about to fight. He agreed to help Dean get five minutes with Sam by attacking Michael the archangel with holy fire. He took this risk more out of love for his friends than out of any real hope that it would help. Consequently, it killed Castiel.
Despite that, this actually turned out to be one of Castiel's better risks. Not only was he resurrected from the dead, but the time he bought Dean to speak to Sam had enough of an effect to allow the latter to regain control of his body from Lucifer. Thanks to this move, Sam and Dean were actually able to save the world.
Infiltrating Heaven
Castiel has had a tumultuous relationship with the other angels. He stirred up a great deal of trouble for them when he first rebelled, again when they had a civil war, and he's hurt as much or more than he's helped.
This put Castiel in the awkward position of having to sneak into Heaven when he needed to free Metatron and get information about the Mark of Cain. While he did get what he came for, Castiel also got on certain angels' naughty lists again.
Trying to Tell Dean About the Apocalypse
Castiel got in way over his head when he learned that Uriel supported the apocalypse. Like any good hero, Castiel wanted to do the right thing and walked right into the danger. He investigated further and learned that all his superiors were of the same mind as Uriel.
Castiel appeared to Dean in a dream so he could arrange a clandestine meeting; however, it never happened. Instead, Castiel was forcibly taken back to Heaven for a sort of re-education to remind him that he didn't serve Dean. He did tell Dean just a couple episodes later, but he was just a little too late.
Antagonizing the Empty
Jack was born with powers he didn't understand or know how to control. Therefore, it wasn't completely intentional when he woke Castiel in the Empty, the place where dead angels go. Castiel accidentally awakened the Empty, which wasn't his fault. However, trying to persuade it to resurrect him was a bold move.
It clearly didn't like Castiel and kept complaining about how annoying he was. This thing claimed it existed before God or Death, but Castiel basically threatened to keep harassing it if he wasn't sent back to Earth. Somehow that actually worked. That's a good thing considering he faced eternity fighting with this thing not knowing what it could do.
Crashing the Car with Hael
Castiel unwillingly became human when the angels were cast out of Heaven in season 8. Metatron stole his grace and gave him all the vulnerabilities of every other human, but with the added problem that Castiel still thought mostly like an angel and wasn't adapted for human life.
When Castiel met Hael, it looked like she may actually be able to help him. This quickly went awry, and Castiel found himself trying to escape from her. To that end, he grabbed the steering wheel while she was driving and crashed the car. With the Winchesters completely unaware of his predicament and Castiel unable to heal himself, he could've killed himself and left Sam and Dean in the dark.
Saying Yes to Lucifer
In a desperate bid to stop Amara, Sam was willing to free Lucifer from his cage. That changed when he discovered it was a set-up and God wasn't the one who told him to free Lucifer. But Castiel still saw it as the best chance to stop Amara, so he agreed to share his vessel with Lucifer.
Lucifer predictably seized control and went about trying to get a prime spot in Heaven and wreck Sam and Dean's lives with his newfound freedom. It's hard to say if Castiel really thought he could handle Lucifer or if he only did it because he didn't see an alternative.