I know no one forced you into this third adoption, but you got delt a shit hand. This is one of those hard-hitting stories where there isn't even a proper lesson to take away from it. You did something good for yourself and good for another creature, but what should have been a happy occasion soured and endangered your pre-existing commitments. You seem like the kind of person to throw out a lifeline to people in need, but when you needed one, none were to be found. What should you have done? Let this cat's behavior drag all of you down into misery and fear?

This experience and memory is going to haunt you for a long time and you should be grateful when it does—it means you still have the ability to empathize. I hope your neighbor finds a good friend in the cat that could’ve been, and unfortunately, he's still technically your responsibility.

If the third cat does end up back in the picture, look specifically for cat rescues in your area and let them know that your situation is dire; they'll usually do everything in their power to re-home your cat if they know that the current situation is endangering other cats.

This is also one of those horrible experiences where someone will one day tell you about some simple solution that they can't believe you hadn't heard of, and it's gonna rip the wound open again. You'll just have to keep reminding yourself "I love all three cats, but one cannot ruin the lives of the other two. I did my best in that situation, and this renewed pain means that I still have empathy."

I'm so sorry that you're in this situation and I sincerely hope that a solution presents itself that gives you immediate catharsis. I wish that angry boi the best and I hope he finds another loving home that has all the right conditions to satisfy a furry ball of aggression.