Isaac B. McCollum was probably not yet a teenager when his family moved from Greene Co., Tennesseee to Harrison Co., Missouri in the 1850s. Missouri was a border state which sent men to fight on both sides. Although Tennessee voted to secede from the Union in 1861, there were many in Greene County who remained loyal to the Union, including Isaac's uncle, William S. McCollum, and several of Isaac's cousins.
Family tradition maintains that during the war, Isaac's father Alexander was taken by northern soldiers to St. Louis and tried "three times as a southern sympathizer, but was never convicted." Isaac is said to have fought as a Confederate while his brother, William, was a Union soldier. On 23 Jul 1864, Isaac was "plowing in a field when two Northern men shot and killed him. Neighbors offered to lynch the murderers but his father... said 'No, it would just cause more bloodshed.' On Isaac's tombstone in Harrison Co. it states that he was murdered and lists the names of his murderers."
An 1888 Harrison County history paints a somewhat different picture:
"Killing of McCollum. - In July, 1864, George Williams, a returned Federal soldier, attempted to disarm Isaac B. McCollum, a Southern sympathizer, and in the quarrel which ensued the latter was shot and killed. Williams received a shot in the leg, but was not otherwise injured. He was indicted for the killing only a few years ago, and after quite an extended and exciting trial, was cleared." 
How can we resolve the conflicting elements in this story? What else do we need to know? What do we need to confirm?
While it may seem elementary, one must first confirm the date of death. Death certificates or registers do not exist for this time period in Harrison Co., Missouri, so the best source will be tombstones. Does Isaac's tombstone really name the people who shot him? The answer seems to be "Yes." Multiple transcriptions of Phillebaum Cemetery near Bethany, Missouri provide a death date of 23 Jul 1864 and state "Murdered by G. Williams & T. L. Sullivan." 
Next Steps (Research Plan):
- Confirm the gravestone inscription, if possible. [In late July, 2011, I attempted to photograph the tombstone, but the cemetery was too overgrown to proceed].
- Newspapers are not known to exist for Harrison Co. for the time of Isaac's death, but it is possible the story may have been reported in other localities such as Daviess Co. or Kansas City.
- Another avenue for verifying the story and determining more details may be Harrison Co. court records.
 Walter Williams, ed, History of Northwest Missouri (Chicago/New York: Lewis Publishing Co., 1915), Vol 2, p. 698.
 Debby Strong Taylor, "McCollum Family History (1724-1984)" (typescript, Gate City, Virginia: December, 1984), p. 19.
History of Harrison and Mercer Counties Missouri from the Earliest Time to the Present, (St. Louis and Chicago: Goodspeed Publishing Co., 1888), pp. 285-286.
 Maxine Taraba, Harrison County Missouri Cemetery Records (to 1984), (Decorah, Iowa: Anundsen Publishing Co., 1985). See also FindAGrave.com and USGenWeb Project’s “Harrison Co., MO Cemeteries” at www.rootsweb.ancestry.com/~moharris/hccem.html, last viewed 9 Jun 2011.