Here is a brother of Susannah B. Magruder Phillips, another son of Fielder Magruder Sr. I kept seeing references to William T. Magruder dying at Getttysburg, though none said which side he was fighting for. Turns out he was one of four West Point graduates who switched sides during the Civil War, resigning his commission and joining the Confederate army after fighting for the Union at the first Battle of Bull Run and the Peninsula Campaign (1862). He was killed while trying to rally troops during Pickett’s charge. He had spent the years since his 1850 graduation fighting indigenous tribes in Minnesota, Kansas, New Mexico, and California.
The first of these links takes you to a basic biography. The second is part of a National Park Service course. If you search “Magruder” on that page, you’ll find an eye witness account of his death.