The former Northwestern professor accused in the gruesome slaying of his young boyfriend took the stand Tuesday and in tearful, often strained tones, said it was his British companion Andrew Warren who fatally stabbed Trenton Cornell that night in July 2017.

       But, Wyndham Lathem told jurors, afterward he felt wracked with guilt, thinking he was also culpable for Cornell’s death — which he characterized as a kinky, meth-fueled threesome gone horribly wrong.

       “I put Andrew and Trent in the same room, I gave Andrew the drugs, I took out a knife without telling Andrew about safe role play with knives,” he said on the stand, red-faced, his eyes squeezed shut. “I brought that drywall saw into the bedroom and I didn’t stop Andrew from hurting him, I didn’t, I hid in the bathroom like a coward.”

       Lathem is expected to be cross-examined Wednesday. Prosecutors put Warren, their centerpiece witness, on the stand last week.

       Wyndham Lathem arrives at a police station escorted by Chicago police on Aug. 19, 2017, in Chicago. (Jim Young / AP)

       Warren testified that he met Lathem in Chicago because they were both suicidal and had a pact to kill each other, but after awhile, Lathem began to speak in general about killing others, and invited Warren to his apartment in the middle of the night because he was going to stab Trent. Warren only joined in the stabbing after Lathem had begun to stab Cornell, he testified.

       By contrast, Lathem — in some of his first words to the jury Tuesday — flatly denied wanting to hurt Cornell.

       “On July 27, 2017, did you stab Trenton Cornell?” defense attorney Kenneth Wine asked.

       “No, I did not,” Lathem said.

       Lathem then detailed the stages of his relationship with 26-year-old Cornell: A Grindr hookup that sparked a friends-with-benefits arrangement and, after a few months, a real romance.

       The two enjoyed BDSM, including knife-play, which Lathem explained to jurors in matter-of-fact tones.

       “It’s the use of knives in a safe, consensual role play,” he said. “No one is meant to get harmed. One enjoys the use of the knife because it’s a little bit scary, it requires trust and there’s a range of sexual experiences that you can get by the feel of the knife on the body.”

       Lathem and Cornell also would use meth before their sexual encounters, in part to boost libido, he said.

       They had an open relationship in which both partners were allowed to seek out other sex partners, Lathem testified. And in the course of searching for people online to discuss BDSM, he was connected with Warren, and the two struck up an online flirtation from across the Atlantic.

       Lathem paid for Warren to fly to Chicago so the two could meet in person. But when Warren arrived, Lathem was disappointed with his looks, and the two did not share much chemistry, he said.

       After a couple of days, Lathem told Warren he wasn’t interested in pursuing a romance, and got Warren a hotel room not far from his apartment, he testified. Lathem then invited over Cornell — who had been slightly jealous of the time Lathem spent with Warren, he said.

       Cornell came over to Lathem’s apartment, then woke Lathem up in the middle of the night wanting to have sex, Lathem testified, saying he invited Warren over for a threesome.

       Warren came over, the three of them used meth, and Lathem told Warren they could use a drywall knife — thinking it would be part of consensual kinky knife-play, he said.

       Lathem was running the dull side of the drywall knife down Cornell’s back when he felt someone else get on the bed, he testified.

       “All of a sudden Trent, he freezes, and a split second later he starts screaming … his head is right next to mine screaming in my ear. He’s screaming in my ear,” Lathem testified, with tears in his eyes. “It’s like this melee that follows, like I’m getting smacked and getting hit and I’m trying to block and defend myself because I don’t know what’s happening, I don’t know what Andrew is doing to him.”

       Lathem said he managed to get out of the bed, drop the drywall knife, and ran to the bathroom, listening to Cornell’s screams.

       “I hear him yell, ‘Help me,’ ” he said, then broke down into sobs on the witness stand. “I didn’t help him, I just stayed in the bathroom and he killed him.”

       The two men fled Chicago after the slaying, sparking a nationwide manhunt that ended more than a week later when Lathem and Warren turned themselves in the Bay Area.

       Lathem testified that he cleared out the drug paraphernalia from his apartment, took some cash and left, believing that police would blame him for Cornell’s murder. He also wiped his iPad, saying he was worried he’d get in trouble for texting about drugs.

       And in the wake of Cornell’s death, Lathem became suicidal, and unsuccessfully attempted to kill himself at least twice, he testified.

       mcrepeau@chicagotribune.com

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