Jacob Shaffer admitted to killing Sidney Wade Dempsey in Huntsville in 2009.
He entered the plea during a court hearing on Friday.
After killing Dempsey, Shaffer went to Lincoln County, Tenn., where he killed his estranged wife and four other people.
The 33-year-old Shaffer reached plea deals that allow him to avoid possible death sentences in all six murders.
Alabama jurors still must consider evidence in Dempsey's murder because Shaffer pleaded guilty to a capital charge.
That mini-trial is set for April 29.