What is the most dangerous ideology in the world and what makes it dangerous?

Religion

Came here to say fascism, but you win. Religion is worse...
- Especially since people use religion to bolster fascism. Fascists love to pander to a nation’s largest religion to lock in their support.
- some say fascism is a form of religion
not necessarily
- Name an ideological concept that has led to more killing and death then religion...
I got downvoted to hell last time I expressed my views about religion, but it has served me well for 29 years now (having been born in the shadow of the Vatican), so I am willing to share again:
I work, collaborate, spend time and even sometimes have sex with religious people, all while maintaining the idea that each one of them is akin to a dormant terrorist cell, and that given the right conditions their fundamental distancing from reason and in some cases recognition of undeserved authority can turn against me and everyone else.
I live a subtly tense life, but usually I am already in a safe place when shit hits the fan.
I also have a very wide definition of religion and of priest.
I'd say monotheism is the real problem.

Any religion. It makes people think of themselves as superior to other groups. It doesn't even matter which religion. They're all bad.

Fascism. It makes people loose their humanity. Fascists endure anything as long as people outside their group are suffering.

American Exceptionalism.

gestures broadly

Nazism seem to be the worst since it's an awful mix of nationalism, imperialism, totalitarianism, state racism, eugenics.

A nazi state basically invades its neighbors and genocide their inhabitants based on race, community or health condition.

So far, only the Third Reich applied it, leading to World War II and the Holocaust, but Japan applied some similar behavior in Asia during WWII.

Bonus point: it doesn't even oppose capitalism, so rich people can still greed.

"Us vs them" mentality.

Religion, because they all believe that they are the only ones who are right, and everyone else needs to believe what they believe, or else something bad will come of it.

TESCREAL

It ends in either replacing humans with AGI or massive atrocities in an attempt to achieve it.

And there are people in positions of real power who believe in this stuff and act on it.

Andreessen posted a manifesto where he said that deliberately delaying AGI is basically mass murder and should be treated as such.

I too, think this is a really dangerous one that hasn't quite pinged on most peoples' radar yet because it's so niche, but like you said, when people with power and influence can actually act on it, they have the capacity to cause a lot of harm over what would normally just be fringe philosophies.

misogyny. I'm not saying it causes the most death and harm (though it may), just that I've never met or heard of any dangerous ideology where misogyny wasn't a core element.

Feminism inoculates against fascism.

The certainty of being right.

Why? Because when (any)one is sure to be right it means the other must be wrong and since they're wrong they probably should not even be allowed to say what they have to say.

I cut a very cherished friend out of my life on Tuesday because of this mentality. When he said "I'm not going to read what you write or consider the point you're trying to make" the first time it was enough. The second time was too much. I expressed my wish for his life to improve, told him good-bye, and removed/blocked him.
The next morning I woke up to a text claiming I didn't value his friendship and that he'd be here for me when I got better. That's when I truly realized how long he'd been disregarding the support and compassion I'd been sending his way.
I spent Tuesday mentally digging a grave for the friendship. I spent Wednesday filling it in, and that night I drank to the loss of a friend and to the health of the man who replaced him.

Look out for number one. Selfishness is at the root of most dangers to humanity.

This mentality and reinforced culture is a big reason inhuman corporate ideals and customs have festered throughout our society like a fungal infection.
It's so difficult to organize because plenty of fellow workers will simply be passive and complacent with whatever increasingly bad lot they're given, or worse, will be straight up class traitors looking to be rewarded by ruthless masters.

Good old fashioned tribalism.

Ignorance.

It's practiced by many, in alignment with arrogance. Many different flags, cloths, creeds and livelihoods depend on it.

I'd say it isn't an ideology, but way too many people take pride on being ignorant. so it ends up being one

Any and every form of dehumanization.

Any and every form of bigotry.

Any and every form of supremacy.

Any and every form of authoritarianism.

Every single person from the homeless guy at the on-ramp to TACO himself is an ordinary human. Some humans are born into extraordinary circumstances, some humans accomplish extraordinary feats, but we are all human. Humans are not born hating other humans. Hate is taught and encouraged and ingrained so that it may be passed on.

Universal Basic Income is a bandaid on a system couched in one of the ugliest human motivations; greed. I chose my name for that reason; Universal Basic Justice. Justice must be the base motivation; not 'eye for an eye' or 'frontier justice,' but the belief in treating others as you want to be treated; the belief in the powers of forgiveness, responsibility and growth; and the power of compassionate care dedicated to help those that are willing to learn and grow.

If and only if all those fail should a human be separated from society in a humane way that prevents their flaws from harming others.

Death Row is not justice. Guillotines are not justice. Systemic violence is not justice. No government nor individual should be empowered to decide who is worthy of justice, of forgiveness, of growth. Of life.

Every human is fallible. Every human deserves the opportunity to recognize their mistakes as well as the opportunity to learn, grow, and make reparations for those mistakes.

I don't see these changes happening in my lifetime. At this point, I'm not sure we humans have enough lifetimes left to achieve these goals.

What I am sure of is the danger and violence incumbent within any ideology willing to look down at another human being.

Neither is this about an ideology per se nor is this quote my own - but I've read this somewhere -

"The greatest threat to our planet is the belief that someone else will save it."

Robert Swan (British explorer and environmental activist)

Individualism.

It has led to a massive amount of duplication of human effort. We could all live massively improved lives if we acted as a community organization instead of a bunch of individual little fuckers whose opinions matter.

You should see what happens when you try force the people of an natural individualistic bend into community organization. Undermining, diversion and later, violence.
- Right. Underscoring that individualism is a highly sinister force.

TESCREAL Because it serves as an excuse for the oligarchs to do whatever they want regardless of the current impact of their actions with the promise of future benefits

Ableism. As disability advocate Imani Barbarin says, if bigotry is the goal, ableism and eugenics are the toolkit. If you look at the history of any form of systemic bigotry, the justification for human atrocities almost always boils down to “well these people can’t contribute to society, so they don’t deserve to be a part of it.”

Not an ideology but I say extremisms, of all kind. Not only religious and political, those are obvious, but also day to day habits.

Any ideology may be dangerous. People who are convinced they're tuned into a privileged view on reality may be willing to kill others to protect it. Human history is full of proofs of that.

"On the whole I think that knowledge is preferable to ignorance, and I am sure that human sympathy is more valuable than ideology." — Kenneth Clark

Any, all and each one of them. The moment you stop looking at how things are, you are finished and set right on the course of bringing unjustified pain and suffering

Beauty. It gives everybody the illusion that they can decide or at least criticize everything. We will end with an unstable system where control becomes impossible and things collapse.