This story reminded me of something similar I had read 10+ years ago.
I'm trying to identify a book of short stories, possibly by the same author. I only remember bits of it.
A story about time travel where the protagonist travels back in time and sees himself as a troubled child staring at him in the dark. The time machine is like some sort of box and you squeeze yourself into it to use it.
A story about a second spaceship landing on a distant planet in search of the crew of the first. But all they find was the damaged old ship and also encounters ugly monkey-like creatures screaming "COPAAK, COPAAK, COPAAK". They defend themselves by shooting down every such creature that they come across. The story is written in the form of journal entries by the protagonist. And after a few days of well written entries, the author starts to lose proper writing style in their words. The last journal entries were "ritttenn lykk theees". And we, the readers, will finally learn that the planet has some serious effect on the crew and they're actually turning into such creatures. Then we realise that the creatures that screamed "COPAAK" were actually the first crew members warning the new ones to "Go Back!".
A story about a white collar criminal who somehow ended up on a strange planet with civilisation as a punishment for his earth crimes. But learns that he's actually given a great job and a second chance to life there. The people appear to be pure and innocent. But after a few good weeks, the protagonist starts to be his old self and begins committing fraud while at his job as a sort of accountant. On one evening looking out his window, he sees red skies and violent volcanic eruptions at a distance and after a while wakes up in the middle of a cult session wearing long clothes and masks. He was the subject of their ritual that day where he was tortured and killed for his crimes against this new civilisation that gave him a second chance at life.
There may be mistakes in my memory of these stories. There may be more short stories in the same book. The authors may be different. I'm not sure.
Please help identify them.