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While being human mostly just meant having a soul that could stay in the world after its vessel died, it was also the only way to keep things going, keep time flowing, and the timeline going. Said humanity could usually make you doubt your place in the world since choices could be made and undone without much effort, provided you had the determination to do it. You could do anything or nothing, and while the choice is entirely yours, there's no actual way to forget what you have already done.
Every run was different somehow, unique really, and at the same time, the same main events would always take place in a similar order. Either live an adventure and help or just live it and kill, there is no better route, because in the end they are all erased because of their selfish need to know more about the world, about every possible outcome, about every possible fight and dialogue. That was the fuel for every single one of the resets and the reason to explore every room every time. While most things would stay the same, the subtle changes were what brought them the most rewarding feelings.
That's how they had found the lab, the determination experiments that had caused so many problems, the core that powered the entire underground and lastly the amalgams that resulted from experiments that should be classified as immoral. Maybe some things were better never being found, but all the same the insight in the monsters’ desperation to survive and get to the surface had been worth several jumpscares and creepy imagery that had come with exploring the true lab. The immediate change in the atmosphere had been unnerving at best, and the disfigured bodies of the once loved monsters had been enough to reset. Sadness and repentance was all there was in those rooms, but all the same hope had never been entirely lost either, if there is a will there's a way and Frisk would make sure that their will was strong enough to help.
Afterwards they had tried to surrender their soul, end the suffering and bring hope but the king would never be strong enough to take it, the toll killing took on him was enough as is having already killed kids their age. In the end, every attempt to make existence easier for the amalgams just brought more anguish and suffering, so while there was no actual way to help them other than trying their hand at research with Alphys nothing seemed to actually do much.
Adding more, determination melted their already unstable bodies while trying to subtract it would just destroy their souls, there was no way to make them completely lucid again and while some monsters might be relieved to be able to see their loved ones again even if they weren't really themselves any more, this fate was worse that actually dying and so their existence was hidden from everyone, even the king. According to the reports these monsters had not been experimented on and were just been taken care of, in the end, the truth was found out.
At least until Alphys had gathered enough courage to face her previous bad decisions and decided to just try again this time with the help of the rest of the people that were at the very least aware of the situation, and additionally (accidentally really since the elevator had failed when they were only supposed to bring some papers) she had let Frisk roam around the whole place uncovering the old notes that recounted the entire determination experiment and many more that were done by the previous royal scientist. The only way to effectively describe it was a really wild ride. The amount of new information was abysmal, and the answers on how to fix their problem were… lacking.
They had the actual data, and they knew the amount of monsters that had entered the facilities, but there were only a bunch of them now. Determination gives humans power over the world around them but it makes a monster's body melt and if they get close to another one they'll fuse and then they lose a piece of themselves and nothing can bring them back from it. Only some could withstand it, and a monster that could actually produce and use DT was even weirder to find.
There was no fixing to do, or at least there were no good solutions that were really on sight or viable. They could only return them to their families and hope for the best, hope that they would have everything they would need to have a good life even if they weren't completely aware of what was happening around them. They still deserved to live with those that they had once loved and still loved them.
This could not have been, if only death wasn't considered such a bad and horrible faith that awaited everyone. If it could breathe it could die. But no, death had to be avoided at all costs, even if the price to pay was you because even then the people that loved you would have you or a vessel that once contained you. Even then that was just dying with extra steps. They would lose you maybe slowly but that would have to hurt even worse.
The amalgams were returned. After the time they spent in the lab, daylight was a novelty for them. And they knew they were going to die a horrible death the day their bodies could no longer handle their own existence but until then they wouldn't be alone. They would cause unbearable pain to whoever thought of the implications of them still being alive after they had fallen down. Not much more could be said since it was really tragic that lives had been affected by it at such a level, this was their best solution.
No more resets would take place in this timeline, not now and not ever since everything that could have been they had already tried, one day they would die and all of their sins would die with them especially every reset born of curiosity that had led to dust and then blood and finally a rope that made the guilt bearable at least for a few minutes until they were back to the beginning.
Hopefully no monster or human would ever have to go through that lab. Maybe life would turn out to be good outside of the underground, happiness was for everyone even those who had lived more than they should, those who had seen every timeline and reset, those that were ignorant. If it lives it deserves a good life.