I don't even know where to start with this.
> Politics is saying mean things about the other side in support of your side.
This is not the definition of politics, this is how it is being executed in some places, there are plenty of places where this is not the default method.
> If you don't believe the other side to be fundamentally bad in some way, why are you fighting them?
This only makes sense if Twitter is your only source of political news/opinion.
You may think the other group is swell but have different opinions on how to get to the same end goal. You may have a disagreement on what an ideal world looks like or what our current one looks like (ie. different perspectives) . None of that includes "fundamentally bad".
See: all parties that have splintered off from a common one. They are still in the same corner but have adjusted their views, strategy or goals. They don't suddenly think their earlier compatriots are evil incarnate.