Well, we certainly wouldn't want that.
Personally, I think Curatolo was Rudy's accomplice. C'mon, here it’s just a reality check.
If Curatolo were an upper class businessman, with a well-paid job, living with a wife and children, a regular social life with his acquaintances and relatives, who would have dinner every night at 9 pm, in a cool apartment in some other neighborhood downtown… then, his profile and lifestyle would appear "not compatible" with a scenario of him testifying for money, doing favors for the prosecution, and hanging out with a criminal.
Curatolo was a 50+-year old who had been a heroin addict and dealer for decades. As a drug addict he was “only interested in pleasure” and completely detached from reality. His proclivity for living on a park bench for ten years can only be described as excessively abnormal. He is a person who would drift around town looking for drugs, and would do God-knows-what to get them. Witnesses described him as being wheeled into the courtroom on an office chair, dressed in a coat, hat and scarf, which he did not remove once on the witness stand. He must have known Guede for the entire time Guede had lived in Perugia. Obviously, he had psychological issues, drug addiction being first and foremost among them. He admitted to using heroin all the time. His companions were other dealers and, no doubt, users of all kinds in Perugia, where police turn a blind eye to the active drug trade.
Your objection is that he was credible because he sounded credible to you, and that he was not brain-damaged because heroin is nothing more than morphine.
You may try to disagree on the interpretation of some of the details listed above, but you can perfectly see the basic data about the person’s profile.
The profile of Curatolo is just compatible with a scenario in which it is likely he committed crimes with Rudy Guede. Every rational person can see that.
Just say it’s compatible and move on.