Yes, I've had to abandon before. I had two cats right before I went into the military. I was in an economically depressed part of the southwest, where animals typically aren't well thought of, either, with a low-paying job at a business that was about to fold, no training or skill to go with a BS, and no money to afford to move to another area. It was the early 90s when the US was in a recession, and jobs were hard to find anywhere, but almost impossible in a small southwest town on the edge of the plains. The Air Force was my salvation and only answer to finding a better life. I had taken in two kittens not long before - they would have died had I not taken them in, one litterally thrown onto a busy highway in a box (and only 6 weeks old at the time) and the other wandering the outskirts of town at only a three months old. They were 4 and 5 months old when I had to leave for basic. There was no animal shelter or humane society in the area, just a few cages at the police station, and "euthanasia" ment a bullet in the head at the city landfill. Try as I might, no one wanted to take on two unsterilized cats, in an area where strays and ferals roamed the streets already. One kindly rancher agreed to take the older female to keep as a barn cat, even though I knew there were risks. She disappeared a week later - her fate might have been the result of a coyote, starvation, dehydration, poisoning, leg-hold trap, or even a rattlesnake bite. The male kitten went to my mother-in-law's house, but she wouldn't let him be a house cat, and he ended up dead in at the street curb a few months later, probably hit by a car.
So, after more than a decade, I still cry when I think about the two cats I abandoned, and can honestly say, that if I had to do it over again, I would have scraped together the money to have them humanely euthanized by a vet before I left! I was too much of a coward to do it at the time, even knowing what the fate of the cats might be. I had no choice but to leave town to find a better life, and couldn't find a decent home to take the cats.
Now, I try to let other people learn from my experience. Humane euthanasia by a trained technician is NOT the worst thing that can happen to an animal, and I recommend that any that absolutely cannot keep their animal, and cannot find a qualified home to take it, should seriously consider euthanasia. That is what I think you should have done, if you could have found a vet that knew enough to eutahnize the animals in a humane manner. They were red-eared sliders, where they not, in a land where they are not native?
Katrina