I've spent a lot of time arguing, and reading arguments, and there's a theme in many of them: people act like their opponents are not human beings.
Disclaimer: please don't nitpick the names I've chosen to talk about these groups.
Men's rights activists, redpillers and other misogynist movements treat women as manipulative and evil, seeking to drain men of their power. To them, women are not human beings, but monsters preying on men.
Tumblr social justice activists don't think of their opponents as being human, either, but nebulous agents of oppression, and do not deserve to be treated with politeness.
Right-wingers fear the government. Left-wingers fear corporations. They're both evil monsters that seek to cause harm to real people.
People used to fear gay people, considering them pedophiles who do nothing but prey on children.
Xenophobes and racists fear people of different cultures and races, considering them "apes" and "barbarians".
Nowadays, terrorists are considered mindless monsters looking to kill everybody.
One of the first things taught to soldiers is to not consider your opponent as human -- it's easier to kill them that way. This applies to arguments, verbal war, as well -- you can argue and insult your opponent all you like, because, they're not human, are they?
Indeed, this dehumanization spreads to everybody we dislike -- if you ever got angry at someone else while driving, did you stop and consider their situation? That they're a human being?
Some people get angry at me when I suggest that certain people are humans. Being human confers on you rights and respect, which we throw out of the window by (subconsciously) believing that our opponents are not human.
It's easy to consider, say, Saddam Hussein as a calculating beast who wanted to kill everybody. But he was fond of dogs, he liked playing volleyball, he took group photos with his family...
That's the important realization we have to make. Anybody, a person who was born, who grew up, who had friends, who ate, who pooped, who got ill, who loved, who hated, who actually lived -- they can all do evil things.
Monsters don't exist. We are all human, good and bad alike.