I had a very similar experience. I got a rescue that was an adolescent (not fully grown) and just the sweetest dog ever. We did all the kinds of socializing tha

I had a very similar experience. I got a rescue that was an adolescent (not fully grown) and just the sweetest dog ever. We did all the kinds of socializing that you're supposed to do but as he got older he became more and more aggressive towards anyone that didn't live in the house. It got to the point where we'd have to lock him up in a room when people came over because we were genuinely afraid that he would seriously injure a visiting friend or family member. We tried all kinds of trainers but nothing at all helped and we finally decided to put him down because it felt like he was a bomb that was just waiting to go off. It was all the more depressing because when there were no strangers around he was a total sweetheart. He loved snuggling up with his humans and playing with our other dog. The experience was so negative that I don't think I'd ever take in a shelter dog again. I paid a good breeder for a purebred after him (my breed choice of Whippet partially inspired by the positive behaviors of my shelter dog) and I've been very happy.