Don't you guys know how the human brain works, especially in the current modern day of "fake news"
Everyone has their own belief, and that belief creates their standard of reality. However, if someone were to challenge their reality, it would create a problem. Since people learn early on that "mistake/being wrong = bad thing", they will do literally anything to defend their reality, because they can't handle the idea that "I might be wrong".
Sure, we can accept we're wrong, but not always.
There are three stages to the "insults" and flame wars you speak of, and it's based on the disagreement.
If someone disagrees with you...
Step 1: They're ignorant. They don't know the truth, and all you need to do is show them the correct information and they'll join your side.
Step 2: They're stupid. They have all the truth and information but can't put it together to get the same conclusion that I do.
Step 3: They're evil. They know that their belief is wrong and are purposely trying to thwart you and push their agenda against a society of people who are stuck at step 1. They are the cause of all our problems.
Everytime we meet someone who does not qualify for a step, we move down the ladder. If they disagree, they're ignorant. If they're not ignorant, then they're stupid. If they're not stupid, then they're evil.
It's just how we as people try to cope with the idea that we may be wrong, which is becoming a serious problem in current society