If there is one thing I've learned about parrots is that there is ALWAYS hope! It might take years and it might take a lot of tweaking here and there but they are inherently loving and forgiving animals and, if you keep at it (love, persistence and patience), they do respond. The response might not be all the way up there on our list of desires (meaning a loving bird that kisses us and shows us his love unconditionally) but they do improve.
Let me tell you another story. Pedro is a 31 year old yellow nape male amazon. He is a wild caught but he must have been caught as a juvenile and not a hatchling so there is no doubt in his mind that humans are not part of his family. He was acquired by a mother and daughter who had a female (also a wild-caught but she must have been younger when she came across people) they though wanted to breed. But they were not friendly to each other (the female was too young and they obviously did not know what they were doing - thankfully!) so they put him in a different cage and that's where he lived the first 27 years of his life, never interacting with humans or the female. Fast forward to the time when the mother became bed-ridden and they couldn't take care of them any longer (the daughter is mildly developmentally disabled) so they gave them to a friend of mine who knew them through her work rescuing cats. This friend loves birds and has experience with small and medium size hand-fed species but none with large aggressive ones or wild-caughts and she mistakenly thought that 'dominating' him was going to take care of his aggression which was a HUGE mistake. She also allowed them to interact and they bonded - but that made his aggression even worse because now, it wasn't only a matter of distrusting humans, it was now his job to 'protect' her and he could come out of his cage and started to fly. My friend has a bad back and is on blood thinners and couldn't move fast enough to avoid his bites and each bite meant lots of bleeding and/or huge bruises so back he went into the cage until, two years after she got them, she asked me to take them in.
Precie (the 33 year old yellow nape female in the story) is now mate bonded to Zeus, a younger (maybe 15), and abused male yellow nape (the owner admitted to 'taking his fist to him' so, needless to say, another one who doesn't 'like' people too much) who fought Pedro for her about ten months after they came to live with me and who, virtually, banned Pedro from their 'territory'- and that did NOT help with his mood!
I've had a hard time with Pedro because, at the beginning, he used to fly out to attack me which caused a number of bad bites - this slowly stopped but he would still come to the very edge of the platform or down from his perch to attack me any time I got close to the cage where he now hangs out with Naida (bluefront 30 year old female) and Mami (old amazon -possibly 50- which I thought was a yellow front but now appears to be a Panama). Pedro would watch me with evil eyes and, as soon as I started walking towards this cage, he would jump down from the perch and standing at the door, prevent me from getting close enough to clean, feed and water so everyday, it was a frigging federal project to get this done. But, thankfully, he is beginning to change and now that the days are getting real short he has calmed down A LOT to the point that, for the last three days in a row, he has not jumped down from his perch so I can do my work without having to trick/avoid him! He still looks at me with the evil eye but now he just bites his rope toy into shreds and, in my mind, I can hear him saying something like: "This is what I want to do to YOU!"

But I praise him profusely, telling him he is a good boy for doing it and, tomorrow, I am going to start rewarding him with a nut for it because, although this is not a behavior that I would normally condone, displaced aggression is a HUGE improvement in his case and another step in the right direction! And it only took a bit over 2 years!
