I mean, there is a REALLY high probability of violence on Monday. There have already been arrests of people that were planing on going and intentionally inciting violence, the dumb**** in the White House is egging them on, a significant number of people that have announced they'll be going are literally banned from Charlottesville, so they have a history of being involved in violence at similar events, and the main organizers of the rally (the Virginia Citizens Defense League) are already saying they have no control over anything that happens and the blame should go to law enforcement if they [ law enforcement] aren't able to prevent violence:
So basically, the VCDL organizes an event that is going to bring several thousand armed gun nuts to Richmond, and now they are claiming, in advance, that they are not responsible for whatever happens. That doesn't sound too good.
If Northam didn't declare a SOE and then there is violence, he'd get (rightfully) pilloried.