The Taiga: After ziplining out of the pioneer camp, walk through the canyon until you come across a small cave. The letter will be sitting on a crate right at the entrance.
Liuda, I don't really hope you'll find this letter, but I will still hide it at the place of our last discussion...
Do not come back. Wherever you end up, it will be better than here. I thought you were making a mistake, I thought you'd be proven wrong. But, by refusing to shoot them, you didn't become cowards. You remained humans. While we, the ones who did shoot... We thought we became stronger, thought we were just, and could do no wrong. In fact, we became monsters.
The marauders used to come often, we've been fighting them off for as long as I can remember. We've been killing them, and I was OK with that. But murdering the surrendering ones, to the last man? I've been trying to wash the blood off my hands ever since... I used sand, I used ash, nothing works... I don't know what's wrong with me, but I'm really sorry I didn't leave with you back then.