This was a very interesting read that got me to go through the whole thing wondering what's going to happen next. Even if it has flaws, I loved reading it and
I think that this suffers from a lack of time. This could've been a lot better if the author was just given more time to do some world-building and develop the characters.
spoilers below
I will say though, the ending is a little... disappointing. Maybe it's intentional. To be fair, I don't really see any kind of happy ending stemming from this. Think about it, what really changed? The MC sacrificed himself not because he was a good guy, but it's because he thought that it was penance for the crimes he committed. Shindo is also still in there. What's going to happen to her now? She is most definitely not going to just let herself die in there. The cycle continues, and at the end of the day, the aliens get to continue their experiment.
I would give it a try if you want a short read.