He may have died from wounds received at Millikens Bend and left in hospital in Jackson Parish on their march to Alexandria.
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Rister, W. C., Private, Company C, 17th Texas (Allen's Regiment) Infantry, Reference Slip, Cards filed with Rister, William M.

Rister, William M., age 20, Private, Company C, 17th Texas Infantry, enlisted May 3, 1862 at Camp Terry for the war, died in Jackson Parish, La. August 4, 1863

M323: Compiled Service Records of Confederate Soldiers Who Served in Organizations from the State of Texas

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The regiment was formed in March 1862 at Camp Terry, Austin, Texas and included men from Belton, Austin, Columbus and the counties of Burleson, Smith and Upshur. It was assigned to McCulloch's, Flournoy's, Waterhouse's, Scurry's brigade, Trans-Mississippi Department. In June 1863 it lost 21 killed, 68 wounded and three missing at Milliken's Bend.
Submitted by: John Heseltine - Historical Data Systems, Inc.

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In May 1863-April 1864, the regiment is attached to the District of West Louisiana, Trans Mississippi Department. On May 31st, at Perkins Landing, the Brigade faces the enemy for the first time driving them back. June 7th McCulloch's Brigade arrived near Miliken's Bend on the Mississippi river and encountered the Union forces. On July 12, 1863, the 17th Texas Infantry marches to Alexandria, Louisiana.
http://www.texansinthecivilwar.com/17th_Infantry/17th_infantry.html

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The battle of Milliken's Bend in June 1863 was the first significant engagement for the Seventeenth Infantry. Milliken's Bend was a victory for the Union, because the Federals stopped Confederate efforts to lift the siege of Vicksburg. In the battle the Seventeenth Infantry regiment suffered the following casualties: one officer and twenty enlisted men killed, four officers and sixty-one enlisted men wounded, and three enlisted men captured or missing.
http://www.tshaonline.org/handbook/online/articles/qks12