Matthew Coleman 'Poured Out His Heart' in Jailhouse Letter to Friend After Children's Killings

Months after he was arrested for allegedly killing his two children with a spearfishing gun in Mexico, Matthew Taylor Coleman has sent a handwritten letter from prison to at least one friend, asking for forgiveness.

"He's really despondent and hopeless," the longtime family friend tells PEOPLE. "He's alone with his thoughts 24/7. He's reflecting on the mistakes he made in life and wondering if there's any chance for redemption."

"He poured out his heart," the friend says. "He begged for forgiveness, but says that he's now where he deserves to be."

On August 7, Matthew and his wife, Abby were packing for a family trip when authorities allege he abruptly put his two kids — Kaleo, 2, and Roxy, 10 months — into his van and drove away from their Santa Barbara, Calif., home.

Police say Coleman drove the children into Mexico. Two days later, authorities say he took the kids to a ranch, where he allegedly killed them with a spearfishing gun and returned to his hotel a few hours later. He was arrested when he attempted to cross the border back into the United States.

According to an FBI criminal complaint which was obtained by PEOPLE, Coleman allegedly told police he was motivated by the QAnon conspiracy theory, which holds the false belief that former president Donald Trump has secretly been battling a cabal of Satan-worshipping pedophiles at the highest levels of political power and influence.

In the criminal complaint, FBI special agent Jennifer Bannon wrote that Coleman claimed to be "enlightened by QAnon and Illuminati conspiracy theories and was receiving visions and signs revealing that his wife, A.C., possessed serpent DNA and had passed it on to his children."

"M. Coleman stated that he believed his children were going to grow into monsters so he had to kill them," Bannon wrote.

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The FBI seized Coleman's computers and phones, and believe that he spent hours surfing and posting on conspiracy theory message boards and groups.

Coleman was indicted on murder charges in September. If convicted, he is eligible for the death penalty. Coleman is now being held in protective custody in a federal prison. He has pleaded not guilty to the charges, and the public defender's office has not returned PEOPLE's calls for comment.

His childhood friend tells PEOPLE that he's unsure if Coleman sent letters to anyone else, but that he was dreading the holidays in jail.

"He said he's sorry, that he never wanted to cause pain, and that he's working through why he made the choices he made," the friend says. "It was a very sad note."