Darren Wilson was not guilty o anything. He was, however, unskilled at dealing with a guy like Michael Brown. He needed some pepper spray or a taser. He could have purchased either or both with his own money. I bet that after this, many cops will do exactly that.
Mace is like teargas, and not something you would use in a confined space like a car.
Even if he had driven away, called for backup and waited until it arrived, before Brown got to his car, this would have resulted in a better job of protecting and defending Ferguson.