The pair are alleged to have broken into the home of the family in Oudenberg, close to the Belgian seaside town of Ostend, on Sunday night.
They are accused of tying up the 12, 15 and 17-year-old children who it is said they terrorised through the night before killing their mother in front of them with a knife, Belgian media, which speculated the ordeal was filmed, reported.
Lapeire, who waited until Belgium's Mother's Day to launch his attack, reportedly told the terrified children: "I am no longer a clinic clown but a crimiclown. That's how I will become famous."
"I am a monster. I know that", he laughed before phoning the children's school to say the kids would be late on Monday.
Lapeire and Haegeman separated and fled. Their victim's body was found in the garage by emergency services on Monday and the children released from their binds by police in a room on the upper floor.
On Monday, Lapeire stood on top of a 13 storey apartment building in the coastal municipality of Bredene, close to his parents' home, waving a pistol.
He was broadcasting on Facebook Live as he threw a gas cannister from the roof. Police snipers were deployed and a local school put into lockdown as Lapeire began firing the gun.
Footage appeared to show him enjoying the attention, he would put the gun on the floor, wave his arms as if surrendering, then pick the pistol up again and fire it. He surrendered after several hours.
His alleged accomplice Haegeman commented on Facebook that he couldn't believe what he was seeing on social media before he too was arrested by police.
The father of the three children told Belgian newspaper Het Laatste Nieuws that he could not find the words to describe what had happened.
"It is too horrible, especially for my children," he said, "all my care and attention now goes to them".
This article originally appeared on the Daily Telegraph.
