It invariably takes two people to cause an accident - one person to break the rules and the other to fail to avoid them. We all see people speeding, not giving way, pulling in front of us etc and all drivers have a responsibility to know other drivers do stupid things then drive in such a way to avoid them when they do. If we drive around with no regard to other road users, then hit them when they break the rules when we could have avoided the collision by driving more carefully, then the responsibility for the accident falls on both parties - one broke the rules and the other failed to avoid them doing so!
It's the same with a fight - it takes two people who are both equally to blame. It's no good one party claiming the other 'started it'. One could have decided not to react and walked away. However, if the second person chooses to fight, both equally responsible (which is why they both get nicked!)