Furthermore, if "the other side" on any particular issue is spewing vitriol and harassing people then your side loses a little by not doing the same because the

Furthermore, if "the other side" on any particular issue is spewing vitriol and harassing people then your side loses a little by not doing the same because the other side's behavior will (at the margins) cause the normal people caught in the middle who might support you to keep their heads down thereby making it look like your side is less accepted and their side is more accepted. So not only will people do it but it is incentivized from a game theory perspective.