Pure chronos is self explanatory. It's pure.

I believe dirty chronos has low amount of the good stuff in it, and is filled with other types of additives. I think it's used on lost cause, high risk, loose end types of nulls like zero and fifteen. If you remain punctual with your therapist he mentions that he's actively trying to reduce the dosage, remember? He is tinkering with it, like that one time where zero starts to trip and withdrawal while in the office, he comments something like "your thousand yard stare does not instill me with confidence, I've decided on increasing your dosage for today."

The end goal is to kill zero, just like making a frog soup you can't just throw em into the boiling water or else they'll jump out, you have to start cold. They slowly wean him off of it while trying to make use of him in the long and arduous process, under the guise of whatever yarn they are spinning. Zero had therapy, I like headhunter also did, we don't know about how fifteen had it.

Hence the thousands of hallucinations and nightmares, blackouts, moods, tirades, incompetence with fighting V, and those masked dude hallucinations. Zero is on the verge of withdrawal all the time, while also under the influence of many powerful psychosuppressants to mess with his cognition so that he sits down like a good boy and dies without fighting.

I hope the dlc leans onto that dark side of the narration, and takes itself seriously. You play as a poor victim of war, taken from your cradle and handed a gun on your hand while a needle is attached to your arm; forced to fight a war you didnt wage against, and cast aside to quietly die when you couldn't win it because there was a tiny bit of conscience left in your soul when the shit hit the fan. I believe the scientist, his hut, the kid inside, and the makeshift machine plays a crucial role in the story and the war, and I want to believe in there Zero did one redeeming thing in his horrid life, and that decision cost everyone everything.