I can tell how stressed and worried you are. I totally get it. I have a feral I've been caring feeding and caring for for over three years now, almost four.
My first thought would not be that he has been killed though. If you put him back outside and he was not neutered, he is going to roam. When I was a kid I had a big beautiful black tom cat, and my mom never had him fixed. He would disappear for days, and a few times for weeks, and I'd be sure he was gone, and then he would re appear. It's possible he could be hanging out in someone else's house? Another cat person maybe who saw him out in the cold and took him in. You have to keep some hope that this is a possibility, and this happens more than you think.
Yes, it is also possible that something happened to him. And you will go crazy trying to think about what, but it's sadly just something we have no control of when we take care of ferals, or we have cats that can go outside. It could very well be that this new tom that has been lurking is keeping him away, that is very possible. The few times that my feral girl did not come home, there was always another cat around that was hassling her. I think about bringing her in all the time, but I have 4 inside cats, and I'm not sure of the health of my feral. Also, one of my boy cats is a bully and she is very timid, and he would mess with her. She does sleep in the garage during the winters, and I live in FL, so it's not too bad for her in this climate.
Don't give up hope. I don't know what else to tell you. It's the sad, sad part of loving these cats that live free and wild. My girl didn't come home for two days once, after I de-wormed her once, with medication from the vet, not over the counter stuff, and it made her so sick, I thought she had gone off and died, I really did. I cried for the two days she was gone and searched everywhere for her. She finally came back the third day but it was over a week before she had her full appetite back. So I know how you feel. There is not much I can say to help you feel better, just know that there are many of us here who have been down that road, and it is really hard. Hang in there.