Lathem stabbed his then-boyfriend Trent Cornell-Duranleau to death in his River North high rise home as part of what prosecutors said was an elaborate sexual fantasy.
Lathem now faces anywhere from 20 to 60 years in prison. His sentencing hearing is in November.
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He was helped by Oxford University employee and British national Andrew Warren.
Warren testified unexpected death of his father in 2016 and torment from his alcoholic sister had "mental impact" and "made [him] feel depressed." He searched the internet to find someone who would kill him and was put in contact with Lathem, who Warren said was also suicidal from losing out on a job.
Warren said the two men made a pact to kill each other: he would shoot Lathem, who would in return "cut [him] open, fatally wound [him]." Lathem flew him to Chicago in July 2017, where they did drugs before driving to St. Louis to unsuccessfully attempt to buy a gun, Warren said.
Lathem then suggested they kill Cornell-Duranleau, Warren said. He had second thoughts, but eventually joined Lathem in stabbing the 26-year-old hairstylist nearly 70 times, nearly decapitating him. He was found in Lathem's condo on the 10th floor of the Grand Plaza Apartments in River North.
Lathem and Warren made anonymous donations in Cornell-Duranleau's name and failed at a suicide attempt after the stabbing, Warren said.
The two men drove cross country to California, where they turned themselves in to authorities eight days after the stabbing, Warren said, adding that he "had enough of being on the run" and wanted to "get everything off [his] chest."
Warren pleaded guilty to the crime in 2019 and is serving a 45-year sentence in exchange for his testimony against Lathem.
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