Two days before killing eight children — most of them his own — in a rampage that shocked the country, Shamar Elkins shared on Facebook what looked like a sweet moment.

A gunman in Louisiana killed eight children between the ages of 3 and 11 in domestic-related shootings at a home in the nation’s deadliest mass shooting in more than two years.

A father, identified as 31-year-old Shamar Elkins, fatally shot his seven children and a cousin, and critically wounded two women, including his wife, in a rampage across at least two locations before...