OF star Patrycia "Paty K" Kratiuk witnessed it firsthand ... telling us on "TMZ Investigates: Killer OnlyFans Model: Deadly Love Story," airing on FOX Monday night at 9/8 Central, how she saw Christian offer up drugs to CC on a platter, urging her to do a line of coke.

Though, it wasn't a shock to Patrycia -- her manager warned her of Courtney's wild ways ahead of their first meet-up in Palm Springs for an OF content shoot ... telling her she was heavily into drugs/alcohol, and advising her to leave as soon as they finished filming together.

Fellow OF model Kristina Santa backs Patrycia's claims ... saying Christian would appear to get a kick out of Courtney being a "little f***ed up" -- this info shedding chilling new light into their violent relationship, and indicating the toxicity between them may have been a two-way street.

Kristina adds, on the surface, they gave the illusion of being a perfect couple ... but things went from 0 to 100 real quick when drugs and alcohol were involved.

But, it was Courtney's drug-fueled rages that always reached boiling point ... with Christian solely being the target of her temper. As you know ... she stabbed him to death in the Miami condo they shared on April 3, 2022, after getting wind he was flirting with other girls during an OnlyFans content trip in Aspen.