
He sent the letter but the letter didn't reach Karel. It only revealed at the last novel chapter. Spoiler.
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Sasya submit himself to the officer as he afraid that Karel's secret as a spy would be found. On the day sasya should be executed, karel came to rescue. He took along all the documents that might be the evidence of Sasya crime. And Karel finally realize what had he done to his beloved. Sasya asked Karel in his hallucinations (he was shocked/traumas by the gunshot) "do you read my letter? Why you didn't bring me flower?". Karel broke to tears. In his letter. "Dear my beloved Karel, I'm leaving for france. I hope when we meet again, I can be someone worth for you".
Explanation : sasya left their house because (1), he has trauma and mentally unstable. Karel left him alone for too long which almost makes sasya become more paranoid so he had to leave the place and so something. (2) he started to chase his dream bwck because that was what makes him happy. (3), by chasing his dream, he would be someone great and make his lover Karel proud. That's why in some chapters show that Sasya push to the limit. He says he only had few years (means until he meet Karel again/karel would released).
Unfortunately, the news about Karel's death just torn sasya apart. And the rumours about sasya slept around hurts karel who was alive and hiding. Their relationship come to worst. Their relationship is super toxic until everything clear out. I could say, both had their flaws. It's toxic but it's a beautiful broken love story.

No he didn't. He just flirting or just let them feel a bit of his body. Sasya has trauma and he afraid of having sxx. He only can accept Karel since Karel had been treating in extra care and gentle in the past. That's why in chapter when Karel order Gudin to sleep with Sasya, the ptsd come to attack and Karel stop his stupidity when realized sasya cannot breath due to his trauma. Poor sasya. I really want to slap karel for doing this to sasya. Also in novel, it says in the last chapter that Sasya has been treated harsly. He just wanted to be loved but society makes him dirty. Slander him doing prostituting. It says half of his act was true. Half is not (true=he indeed flirting and seducing old fart to get principal position.. false=he didn't sell his body like sxx)
Note: sasya ptsd starts before he met karel (his family was killed by grand duke who was crazy over him. When he ran away from than man, all his remain family were killed so saysa blame himself for that). That's the start of his mental illness. And it getting worse after he got second trauma (the robber attempt to rape him and he accidentally killed that guy). I feel sad for sasya..

Okay gather here if you were one of those hating on Sasya, like wtf, he had his trauma way before Karel came to the picture, so you don't expect that kind of person to just heal over night just cuz they found love, some people take years in healthy relationship to even start the process of loving themselves, and this explain a lot about Sasya behavior. Thank for clearing the air.

Trauma still doesn’t justify hurting people and using someone/manipulating and I did dislike him at first but did not shame him for what he did (sponsor etc).
Karel became worse during their temporary stay at their new place in the village. (Keeping Sasya locked in etc and now even after they met he’s insecure and still locking him).
But his trauma is the reason why he’s like this. It’s not his fault he’s like this. And reading the story fully caught up, it’s clear Karel is the master manipulator now because he’s afraid his possession will abandon again (even though he doesn’t know that Sasya didn’t abandon him, both of them suck at communication).
Now that most see the big picture, most understand Sasya. We only saw Karel’s POV at first and I judged based off that.
Big issue here is trauma and miscommunication.

Honestly your right alot of people were judging sasya for his mess but I didn't actually, if you look very well people who are traumatized and having heal tend to do a lot of weird and fck up things, in the sasya case he wasn't enlightening, didn't know much about love till he met Karel, he was unsure and insecure about the love at first, who won't when people in your life just come in and use you as they please, though his actions wasn't justifiable but it's understandable now.
And for Karel, that dude is still deeply wounded, I just hope sasya can be smart enough to clear the air before we all are driven into another chapter of tears.
And low-key I like that Karel is obsessed with him, though it's bad, but that means he still has feeling for Sasya, just to scared to let them free.
I don't remember if it has been explained or not but i don't understand why Sasya never wrote to Karel and decided to leave. I don't remember the details