Its the media (or more generally group dynamics) too. One side exaggerates how nasty the other side is, which gets people outraged and engaged, then they act nasty to them because its 'justified.' It also creates a feedback loop where regardless of what the other side did originally they are now much more likely to be nasty to your side simply because you were nasty to them. Maximizing engagement like this necessarily destabilizes discourse. There is a huge monetary incentive to maximizing engagement, while there is only a mild philosophical one to stabilizing discourse, so guess which one loses.