The two sides aren't capable of proper communication. Obviously nobody thinks they are 'evil', so the actions of both sides feel justified. Take a topic like world policy (which everyone has an opinion on) and the discussion is just emotional outbursts, name calling, appeals to nationalism and so on. You can look up videos of 911 truthers debating the non-truthers and eventually one side starts patronizing or abusing the other (both sides equally guilty of this).
The topic is just too charged and people are too emotional and self-righteous for that to be possible. It's basic psychology. The fight or flight response doesn't only apply to physical threats. The amygdala (primitive part of the brain which controls emotion) gets triggered when your beliefs are challenged as well. At that point people will either put up the ignorance shield and try to get away from the argument (flight) or get aggressive about it.http://en.wikipedia.org/wiki/Amygdala_hijack
It takes a lot of practice to train your prefrontal cortex to catch that moment.
The Truthâ„¢ is always somewhere in the middle, not on either ends of the extreme. So both sides are inherently wrong and can't "prove" anything. For each legitimate argument, they discredit themselves with 10 other, basically.
Also, what you said about squashing one theory and another popping up. If you ever listen to one of these debates.... the first guy might make a point, the other guy will point out why it's invalid and then the first guy will just say "well what about...(something irrelevant to the first point here)" and they can just keep going on and on.