Dear Bill--I recently found this genealogy for the Chapmans of Morgan County Missouri, parents of Nelson Knight Chapman, in case it is helpful. Still does not answer the question of why he went by "Bill."
Seems like, since he was garnering a dangerous reputation to hold in Missouri, you would not think he would use his father's name. After taking a long hiatus, we are getting back to wrapping up the Kavanaugh manuscript. http://boards.ancestry.com/thread.aspx?mv=flat&m=380&p=surnames.wainscott
2. WILLIAM4 CHAPMAN, SR. (JENNY (WAINSCOTT)3, ISAAC2 WAINSCOTT, SR., RICHARD1) was born 1793 in Madison County, Kentucky, and died 04 Oct 1858 in Morgan County, Missouri. He married ANN "NANNIE (KNIGHT) CHAPMAN 24 Feb 1814 in Gallatin County, Kentucky, daughter of WILLIAM KNIGHT and BEULAH KNIGHT. She was born 1797 in Gallatin County, Kentucky.
Children of WILLIAM CHAPMAN and ANN CHAPMAN are:
3. i. JOHN SHERMAN5 CHAPMAN, b. 19 Nov 1814, Henry County, Kentucky; d. 1865, Roxbury, McPherson County, Kansas.
ii. JEMIMA (CHAPMAN) SHAW, b. 1817, Gallatin County, Kentucky; m. SAMUEL SHAW, 15 Sep 1844, Gallatin County, Kentucky.
4. iii. WILLIAM CHAPMAN JR., b. 11 May 1819, Gallatin County, Kentucky; d. 18 May 1901, Bloomfield, Stoddard County, Missouri.
iv. NANCY (CHAPMAN) WILLIAMS, b. 1821, Henry County, Kentucky; m. LEMAN F. WILLIAMS, 23 Apr 1840, Henry County, Kentucky.
v. MARY JANE (CHAPMAN) CLARK, b. 1823, Gallatin County, Kentucky; m. ARCHIBALD M. CLARK, 22 Sep 1840, Morgan County, Missouri.
vi. ELIZABETH (CHAPMAN) O'NEILL, b. 1829, Kentucky; m. THOMAS O'NEILL, 15 Jul 1843, Morgan County, Missouri.
vii. RICHARD P. CHAPMAN, b. 1831, Gallatin County, Kentucky; m. (1) MARGARET (MORRISON) CHAPMAN, 17 Jul 1850, Morgan County, Missouri; m. (2) SARAH ANN (MOON) CHAPMAN, 25 Dec 1867, Morgan County, Missouri.
viii. CHARLES K. CHAPMAN, b. 1834, Kentucky; m. REBECCA A. (IVY) CHAPMAN, 02 Dec 1852, Morgan County, Missouri.
ix. JAMES HENRY CHAPMAN, b. 1835, Kentucky; m. CHARLOTTE (TEVIS) CHAPMAN, 10 Sep 1854, Moniteau County, Missouri; b. 1835.
5. x. NELSON KNIGHT CHAPMAN, b. 08 Nov 1837, Near New Castle, Henry County, Kentucky; d. 01 Jun 1917, Windsor, Henry County, Missouri.