You know better than this. Anderson's orders were to hold all the United States forts around Charleston, and to use his judgement about how to do that. He felt that the best place to hold with his meager force was Sumter. But it's absurd to say that he seized it when it was already under his command.
However, since you defer to Anderson's orders and say: "He felt that the best place to hold..."...
Of course that was not his order.
With regard to the word "seize" with which you seem to have a problem, there was an armistice in existence that prohibited military improvements by either side. He was not authorized to move his command to the neutral site, where he established a military post, pointing cannon at downtown Charleston.
That was seizure, and the people of the time knew it.
Commentary of the time is more accurate than your opinion....
Anderson's move to Sumter did not play well with South Carolinians. From page 8 of the Jan. 1, 1861, New York Times, reporting an article from the Charleston Courier of Dec. 28, 1860. (Italics as reported in the Times):
The newspaper offices were besieged, the hotel halls were thronged, and even the grave and serious gentlemen composing the State Convention shared in the general excitement. On all hands anger and indignation was expressed at the supposed perfidious conduct of the Federal authorities, at whose instance it was first thought the movement was made. The people were greatly incensed at the idea of a willful breach of those assurances of non action which had been volunteered by the Government at Washington and upon which so much reliance and confidence had been placed by the entire population, that every impulse to take the necessary precautions for their own safety had been restrained.
Instinctively men flew to arms. Orders were immediately issued to the following Companies to hold themselves in readiness for service: Washington Light Infantry, Capt. C. H. Simonton; Carolina Light Infantry, Capt. B. G. Pinckney; Meagher Guards, Capt. Ed. McCready, Jr.; altogether forming a portion of the Regiment of Rifles, commanded by Col. J. J. Pettigrew and Major Ellison Capers; also, to the Marion Artillery, Capt. J. G. King; Lafayette Artillery, Capt. J. J. Pope, Jr.; Washington Artillery, Capt. G. H. Walter; German Artillery, Capt. C. Nohrden; all under command of Lieut. W. G. De Saussure; Adjutant, Jas. Simmons, Jr.; Sergeant-Major, E. Prioleau Ravenel; Quartermaster-Sergeant, J. R. Macbeth; Surveyor, A Barbot: Surgeons, P. Gervais Robinson and Middleton Mitchel. Also, the Palmetto Guard, Capt. Thomas Middleton, and Cadei Riflemen, W. S. Elliot.
All the military forces thus ordered out promptly obeyed the summons, and the streets were soon enlivened by the appearance of individual members of the different organizations in their uniforms.
Maj. Robert Anderson, U. S. A., has achieved the unenviable distinction of opening civil war between American citizens by an act of gross breach of faith.