I went back to the car sales room, gave them my card and a flyer, and told them that if a dog was becoming a nuisance, they could call me and I would relocate the animal for them. They said it wasn’t them, and I said I knew they wouldn’t be so stupid, and that I hoped they would tell the other car sales lot owners, who obviously were morons.
I would suggest speaking to him face to face, nicely (because the ultimate goal is to stop any more killings rather than seeking retribution), and point out that many people are angry with him because they believe he is poisoning their pets instead of putting up a fence, and that everyone is asking where he lives. Tell him that you are worried for him, and that, to let people know it’s not him, perhaps he should put up a fence to keep dogs out. Maybe casually ask who owns the land and enquire as to whether he has a legal right to farm there, as the enemies he is making must never find out if he is illegal, as they will cause him all kinds of trouble. Perhaps one of the dogs belonged to a gangster. You get what I’m saying.
Let him know you want to help him because you worry what might happen to him if another dog gets poisoned. You could introduce yourself by looking around his plot for telltale signs of poisoned meat, and tell him bluntly what you’re looking for. Tell him your neighbours are worried that their kids might accidentally get poisoned. Ask him if he knows which farmer did it and where the farmer might live (people behave very, very differently when you take the battle to their front door, or even think you may). Take some friends to help you look for that poisoned meat, just so he knows you’re not the only one who is concerned for other dogs’ and his safety.
Really no need to be overly menacing. Just let him know that angry people are looking for the poisoner.
As you can’t be sure it’s him, this is certainly the best route.