Everyone understands mindless violence, I think; even violent murder, but spitting is a sub-human act.
Violence is an understandable - even honourable - human reaction to this sort of provocation. Spitting is just vile and horrid. The minute you spit at someone, you lose the moral high ground*.
*Although Franck Rijkaard spitting in Rudi Voller's perm was slightly funny.
* And Ruddock deserved it for getting involved.