Randy Walker told a news conference that until the pardon, Barbour was someone who he would have supported for president, the Biloxi-Gulfport (Miss.) Sun-Herald reported Tuesday.
Walker and three other family members whose relatives were victims of David Glenn Gatlin expressed their outrage at Barbour's action.
Gatlin drew a life sentence after he was convicted of killing his wife, Tammy, in 1993 while she held their 2-month-old baby. He then shot Walker, a family friend.
"The governor himself ought to have to look these families in the eye and say 'I'm letting this guy out.' He took the coward's way out," Walker said.
Tammy's sister, Tiffany Ellis Brewer, said learning of the pardon was "like it's happening all over again."
In recent days, Barbour has pardoned five convicts who worked as trusties at the governor's mansion.
Democratic lawmakers in Mississippi are calling on the legislature to restrict gubernatorial pardon powers.