I work in addictions and definitely agree that some people can keep up a reasonable facade of normality and respectability, even when using large amounts of drugs. However, this is usually because they have money and very good social and family support - and often because they are still reasonably young. However, even in the most respectable looking people, there is usually huge amounts of chaos if you scratch beneath the surface of their lives.
However, Curatolo didn't have money and didn't have any social support - and was in his 50s and was definitely not someone who was still managing to live a productive and in any way reliable life. Twice a day he would have started to feel withdrawal symptoms kicking in and then there would have been the scramble and chaos of finding more heroin. He would unlikely still be smoking heroin as this just wouldn't be affordable and injecting takes a huge toll on your body. As a long term injector of heroin, he would likely have vein and nerve damage, possibly be Hep C +ve, possibly had bacterial endocarditis, likely suffered from blood clots and pulmonary emboli - usually by someone's 50s, their body is not in a good way, which would likely explain why he died shortly afterwards at such a young age.
It's also rare that people using heroin will only use heroin - this might be possible if you have enough money to always have a large supply of heroin. However, if living on the street there was likely days he couldn't get enough of what he needed and instead would drink heavily, use cannabis, benzodiazepines - whatever he could get his hands on to stop the withdrawals from getting too bad.
Even saying all this, being a heroin addict in itself does not make you an unreliable witness. If he had reported to the police the day after the murder that he had seen two young people in the square, accurately described them and then picked them out of a line-up, his testimony would likely be believable. Instead you have him coming forward a year later, which would have been after seeing pictures of Amanda/Raffaele in the papers daily - and claiming super human feats of memory, even though he would have seen them in the dark, had never met either of them, had no reason to preseve this memory and was obviously even confused about the date when questioned. He would also be likely to tell the journalist, prosecution whatever they wanted to hear, if they gave him enough money to get his next heroin