My friend keeps killing his pets intensionally

I've been living with a friend for the past three months because my house is undergoing major construction work. Now, here's the issue that's been eating away at me. A few years ago, my friend adopted a cat. She stayed with him for about a year, and then he told everyone that she ran away. Later, he confessed to me privately that during a depressive episode, he did something horrible. He strangled the cat. He realized what he had done too late, and she was gone. This was shocking to hear and completely changed my opinion of him. However, we’re also professionally connected, and cutting him off wasn’t an option. Over time, he occasionally expressed guilt about what he’d done, and I would brush it off with comments like, “Okay, fine, now don’t kill another one,” just to defuse the tension. Fast forward, he bought a Persian kitten. For about a year, he took care of her as she was still a baby. Then, when he bought his own house and moved in, he felt like the cat might be lonely. He decided to adopt another, a year-old male cat who had been abandoned by his previous owner after they had a baby. This male cat is playful, affectionate like a dog, and loves attention. My friend appreciated the affection but not the playful nature, especially when the cat would break things around the house. Living here, I’ve seen some disturbing things. My friend has stopped caring for the cats entirely. He doesn’t feed them, doesn’t bathe them, and cleans their litter once a month at best. But his treatment of the adopted male cat is especially alarming. Whenever the cat breaks something or makes noise at night, my friend beats him with a wiper as hard as he can. He also locks him in a small ventilation area, which is open to the ceiling, leaving the cat exposed to the cold. There’s no food or water in there, and it’s covered in poop. My friend often tells me not to feed the cat for a day or two as a form of punishment. When I protest, he dismisses it by saying, “He’s an indie cat; he can survive without food for days.” One time, when I wasn’t home, the adopted cat injured his leg. I told my friend to take him to the vet immediately, but he ignored it, saying, “It’s actually good for me; now he’s not running around.” When I came back, the injury had healed on its own, but the cat was left with a permanent limp. My friend now walks around joking about breaking the cat’s other leg to stop him from “destroying things in the house” or even killing him outright. Knowing his past, this terrifies me. For the past three days, the adopted cat has been locked in that small ventilation area without food or water. It’s freezing outside, and I can’t stand seeing him like this. I’ve tried stepping in by feeding and caring for the cats when I can, but my friend keeps shutting me down. I’ve even offered to take the adopted cat myself, but he refuses. What makes this more complicated is that my house is almost done, and I’ll be moving out soon. I’m genuinely scared about how he’s going to treat the cats, especially the adopted one, once I’m not around to intervene. I’ve thought about secretly rehoming the adopted cat, but I feel conflicted. My friend has let me stay in his house rent-free, and doing that would feel like betrayal. But at the same time, I can’t ignore what’s happening here. I’m torn and don’t know what to do. How can I handle this situation without betraying my friend but still ensure the safety of these innocent cats?