Reactive to any animal

I’m not sure what to do at this point. I’ve raise my dog (chow mix) since he was a puppy and put him in training to help with his aggression but I’ve hit a dead end. He’s attacked farm animals, cats, and most recently a friends dog. I feel like it’ll be near impossible for him to find a home that won’t have these triggers and i’ve started to think of euthanasia since he has killed other animals. I’ve never made this decision before and I’ve never considered it before now. Is this the best thing to do for him and for the safety of other animals?