Family Source: Oklahoma Dentist Accused of Killing Girlfriend's 19-Month-Old Son 'Was Living a Complete Lie'

"Everything was a lie," says a family source, who alleges Bert Franklin had two Facebook pages

An Oklahoma dentist accused of killing his girlfriend’s 19-month-old son is being described as “not human” and “a monster” by a source close to the child’s mother, who spoke with PEOPLE Friday on condition of anonymity.

Bert Franklin, 35, a married father of four, allegedly dropped his girlfriend’s son, Lincoln, on his head before kicking him, police have said. The child died from his injuries two days later, according to police.

Part of the alleged abuse was captured by a home surveillance camera. However, the footage does not show the entire incident. Lawyers for Franklin have said he was kicking a dog and not the 19-month-old boy.

After allegedly kicking the child, Franklin can be seen in the footage cradling the baby, who appears limp, and eating a slice of pizza.

Lincoln’s mother, Roxanne Randall, was home at the time of the alleged incident, but told investigators she was upstairs. She ran to her Tulsa home’s basement when she heard a loud thud, but Franklin allegedly told her he was throwing a ball to the dog and the baby was asleep.

“He is not a human – he was eating pizza while this baby was unconscious,” says a source close to Randall. “He’s a monster. He is absolutely a monster.”

According to medical examiners, Lincoln – who was born four months premature – sustained a skull fracture as well as brain bleed. He was transported to Tulsa Hospital where he was pronounced dead. His mother knew something was wrong the day after the alleged incident, when the child – his eyes, wide open – started having trouble breathing, the source says.

When asked how Randall has been handling the loss of her child, the source says she’s “just existing at this point.”

The source tells PEOPLE Randall trusted Franklin with Lincoln, noting the couple had been dating for nearly a year. Randall, the source says, was unaware Franklin was still married.

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“He had two Facebooks, for God’s sake,” the source says, saying he blocked his wife and people she knew from one of his Facebook pages. “He was living a complete lie. Everything was lie.”

The source tells PEOPLE Randall was led to believe Franklin had “legally separated from his wife and was going through a divorce.” Randall’s friend says Franklin never seemed capable of such a horrific act.

“Even Lincoln’s paternal grandmother and Lincoln’s father’s family adored Bert,” the source offers. “I can tell you nobody saw this coming. Not a single person.”

Attempts to reach Franklin’s wife and family for comment were unsuccessful Friday. Franklin has yet to enter a plea to the murder and fraud charges filed against him, and is being held without bail.

The source tells PEOPLE Franklin engaged in a texting war with Lincoln’s father, who’d been texting Randall to spend time together.

“Lincoln’s dad was calling … for a couple of months, wanting to hang out and such,” the source says. “Franklin wasn’t happy about this and texted Lincoln’s dad, threatening to castrate him. He said he was going to ‘Game of Thrones [you] if you keep calling my girlfriend.'”

The source says Franklin gave Randall no reason to be suspicious of him, and that the grieving mother is still trying to wrap her head around his arrest.

“Lincoln was the happiest baby I have ever met,” the source says. “He never cried…he was just happy existing. As long as he could see somebody, he was happy.”

Lincoln was a fighter, according to the source, who tells PEOPLE the child endured months of emergency care before going through a number of surgeries and therapies.

“It’s just disgusting,” the source says. “Who would commit such a violent act?”

The source says Randall “saw Lincoln through a lot of medical issues, so for him to be taken out by a grown man like this is very sad. It’s very disheartening.”