Interesting question but a simple answer, people just hate other people, reason can be simple or complex, rational to them or completely paranoid nonsense, doesn't matter. There is no reason or rather lots.
Someone above mentioned Hindus and Buddhists as not hated, that question depends on who you ask, they too hate each other and others hate them, check the news. No group or person is exempt from this universal human trait.
In America today there is much hatred of government, the very institution that defends and protects them. Figure that out.
Look at homophobia, or Hillary Hatred, Or immigrant hatred? Or racist hatred? Now transgender people are hated by many.
I do think and experience hatred as stronger in the closed mind, in the conservative mind, in the narrow view that this is right and they are wrong.
It comes in varying levels and degrees and I think 'Hatred' gives meaning to people's lives, it gives them focus, it comforts them, it allows shared feeling and commitment.
That's the harder question, why does it fill the hearts and souls of so many people? In America this election has brought its ugly head front and center.
"The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors so that, in glory and triumph, they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of this pixel on the scarcely distinguishable inhabitants of some other corner, how frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds. ¶ Our posturings, our imagined self-importance, the delusion that we have some privilege position in the Universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves. ¶ The earth is the only world known so far to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment the Earth is where we make our stand. ¶ It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we've ever known." Carl Sagan
"Conservatism is the theoretical voice of this animus against the agency of the subordinate classes. It provides the most consistent and profound argument as to why the lower orders should not be allowed to exercise their independent will, why they should not be allowed to govern themselves or the polity. Submission is their first duty, agency, the prerogative of the elite." Corey Robin 'The Reactionary Mind'
"The American far Right, with its white supremacism, fascination with guns and explosives, profound hatred for the Federal government, poses the biggest terrorism threat in the country by far. Worse, they receive support from right wing media in the US because they hate taxes just the way the ultra-conservative media moguls do. Although their attacks are ideological, directed at civilian victims and violent, they are seldom categorized as ‘terrorists.’ Rather what they do is “mass shootings.” In the meantime, plots hatched by Muslims in the US during the past few years appear almost always to be a form of entrapment by the FBI."
Top 3 White Terrorist Attacks in America this Week
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We first kill people with our minds, before we kill them with weapons. Whatever the conflict, the enemy is always the destroyer. We're on God's side; they're barbaric. We're good, they're evil. War gives us a feeling of moral clarity that we lack at other times." Sam Keen