This new info on his being at Ft. Jackson leads me to believe that he had some "medical"(psych maybe?) issue which was being investigated while he was in the NG. He likely was pulled from his Basic and sent for medical examination. If he had simply washed out of Basic, the Army would have discharged him then and there from Ft. Jackson with a ticket "home" to wherever he lived. With medical, especially Psych issues, they would want to test, counsel and confirm before discharge. This opens up a lot of possibilities as to his behavior and his heinous act!
Ft. Jackson was where I had Basic, and my Dad and Grandfather as well! We had 22 of the 52 starting wash out in my platoon. A couple more were retained for Psych evals for attempting suicide, and 2 for "deviant" behavior. Those were gone the next day.