Angels aren't typically portrayed as villains, but Supernatural really goes all-out in imagining the horrific side of angels. These hyper-powerful beings often act more like monsters, and their natural lack of emotion seems to prevent them from regarding human issues with any seriousness.
The angels in this show go so far as to call humans "mud-monkeys," which just about says it all. Here are some of the most horrific things that the angels of Supernatural have done to put them squarely in the monster category, along with vampires, sirens, and wraiths.
Killed Possessed Humans
Sam and Dean are in a bind when it comes to icing demons. The demon blade will work, but it will also kill the demon's host. That's part of what drove Sam to practice his psychic exorcism abilities when Ruby was feeding him demon blood; he saw it as an opportunity to save the person the demon was wearing.
But angels shouldn't really have this issue. It would be easy for them to overpower a demon, exorcise it, and then heal the vessel. But they don't. It's more convenient for them to just kill the demon and the human at the same time, so they never bother. They don't bother to resurrect the humans, either. And it's not just demons, they kill angel-possessed humans too.
Created Demons
Credit for this one must go strictly to Lucifer, but it's a doozy. There's no knowing what would've happened to sinful humans if Lucifer hadn't started the demonization practice, but after he warped Lilith into a demon there was no going back. It set a precedent for all human souls that went to hell to become the black smoky terrors that Supernatural fans love to hate.
And it's not even just evil humans this happens to, anyone who makes a demon deal goes to hell, no matter what the deal is about. That includes Dean's deal to save Sam, and fans will remember the horror of a prospective demon Dean.
Killed Humans To "Help"
Humans are generally in agreement that suffering is bad. However, that does not necessarily mean that people would rather die. In season 9, after the angelic expulsion from heaven, an angel named Ephraim tries to keep doing his old job — badly. He used to euthanize angels on the battlefield, but once on earth, he starts killing humans who are suffering, no matter what kind of suffering. He makes them explode into pink goo.
This one can be chalked up partially to misunderstanding, but if angels are immortal beings, how did they never bother learning these things about humans? And it should be noted Ephraim wasn't the only one to think of 'helping' with death; Uriel thought smiting a whole town was for the common good in season 4.
Trapped Human Souls In Their Own Heavens
Sam and Dean were a bit busy running from angels the first time they remember visiting heaven. But when they had time to be disappointed, one of those disappointments was the concept of separate heavens. Not that the idea of a personal heaven is bad, but being permanently separated from all your loved ones kind of sucks.
The angels don't have to keep it this way, especially since Chuck left the building. But all the human souls are still stuck in their own prison-cell heavens for eternity, with nothing but the memories of loved ones who could be in heaven with them.
Altered Time
It's hard enough for humans to fight superpowered beings, but altering time in the fight? That's just a low blow. This happens multiple times, like when Anna tries to go back in time and kill Mary to prevent Sam's birth (or conception, for that matter). But then Castiel and Balthazar tried that crap to create new souls that weren't meant to exist, just to gain some power.
This resulted in Fate having to go around murdering people that weren't meant to exist in the first place. While some of the alternate timelines may seem appealing, fans are well aware there's always a catch-22, and the angels shouldn't risk that kind of chaos.
Abused Their Vessels
All the angels have to take vessels to walk the earth, but some are more respectful to their human hosts than others. Raphael leaves his vessel like a vegetable, and few of them seem to actually explain what humans are getting themselves into when they consent, including how long it could last or the kind of physical wounds they may endure.
But the worst abuse would unsurprisingly be Lucifer's. He used his vessel to create the nephilim Jack. It's reasonable to assume that his vessel didn't consent to make a baby hellspawn when he said yes.
Lied to Subordinates
Throwing your subordinates under the bus is always poor management and a jerk move, but it's a lot worse when you're telling them that the orders are straight from God. Castiel was one of many angels who thought he was trying to stop demons from releasing Lucifer from his cage, and many died in the process.
But Uriel, Raphael, and other higher-ups lied about the apocalypse, leading their subordinates to think that they were trying to stop it when they were really just pawns to make it start. Add to this that most angels' moral system is based more on obedience to superiors than any kind of actual ethic, and this is one vile manipulation.
Civil War
After the smackdown with Lucifer and Michael didn't go as planned, the angels dealt with a power vacuum. As bad as their higher-ups were, they weren't really trained in this whole decision-making thing, and someone had to fill the vacuum left by both God and Michael.
It seems like immortal beings that considered each other family and had eons of experience would be able to figure it out, but instead they just divided into factions and started the slaughter anew. They let the humans get caught in the crossfire, even releasing the heavenly "nukes." To top it off, this was one more step toward becoming an endangered species by season 15.
Started The Apocalypse
Normally the heaven vs. hell thing is just that — angels against demons (and Satan). But in Supernatural, heaven has gained a few Lucifer sympathizers by the time the apocalypse rolls around. So instead of trying to stop it, the higher-ups decide to help it start. They don't care how many humans die in the process, either.
They know exactly what's going to happen if Sam kills Lilith, but they actively prevent anyone from telling him. Sam doesn't want to be Lucifer's vessel, either. But instead of protecting him or helping him avoid that, they do everything possible to coerce Sam and Dean both to serve as vessels. If they were protecting humanity, they would've genuinely fought to keep the cage shut.
Nothing
Before they knew who Chuck really was, Sam and Dean's biggest beef with God was his non-intervention. He knew there were monsters covering the earth and that demons were trying to start an apocalypse, but he didn't appear to do anything. The same could also be said of the angels. It was stated that they weren't meant to take vessels until the apocalypse, but that doesn't mean they couldn't have done anything.
They've been shown to appear in human dreams, for one. And they could've focused on helping earth after Michael and Lucifer were in the pit. They really don't seem to care that the earth is covered in monsters or that those monsters can actually turn humans and prevent them from going to heaven. So...what's their excuse?