Nikolas Cruz chuckled in the video, saying: "You're all going to die".
Zachary Cruz said he wonders whether he could have stopped the carnage at Marjory Stoneman Douglas High School.
Scot Peterson resigned from his post a week after the February 14 school shooting.
The high school junior and history teacher had a lengthy spat on Twitter earlier this week.
The chemistry teacher was arrested for allegedly failing to safely store a firearm and later released on a $250 cash bond.
"I don't blame a kid for taking a weapon to school, quite frankly, these days."
Zachary Cruz said he wanted to "reflect on the school shooting and to soak it in."
Frantic calls to emergency services during massacre that left 17 dead reveal chaotic scenes.
"While it may have been a violation of policy to not notify their supervisors that they were going there, their intentions were brave and heroic, I think," an officer said.
Eighty percent of Republicans agree with arming teachers, while 88 percent of Democrats oppose it.
Republican Kelli Stargel opposed a ban on assault weapons.
Police responded to Dalton High School after receiving reports of shots fired around noon. Investigators discovered a teacher barricaded inside a classroom.
Erick Erickson lashed out at David Hogg, a student who survived the Parkland school shooting.
Many social issues divide Americans by age. Gun control is not one of them.
The gun rights group has its own lobbying arm and employs in-house lobbyists.
Wallenpaupack South Elementary School will temporarily move its students while a controversial nearby church hosts an event that is expected to involve semi-automatic rifles.
Children experiencing depression, anxiety, and fear following the Parkland high school massacre have been flooding Florida mental heath faclities.
"Keeping guns away from dangerous people and people with mental issues is what we need to do," Scott said.