Drug addiction. (Alcohol, too, but that’s secondary once drugs enter the equation, and it only exacerbates the drug addiction — in other words, consuming alcohol becomes about increasing [or decreasing] the effects of the drugs, not about simply getting drunk.)
Until a toxicology report is released, we can only speculate about the nature of that drug addiction, but given the items that were found in his room, it’s pretty clear that he was at least smoking crack — and that he was an experienced user.
Also…
This has nothing to do with him being a singer. Humans in general do this. Admitting to needing help with a school or work assignment is difficult enough for most people because we don’t want to be seen as “weak” — now imagine being Liam Fucking Payne and working up the courage to admit to needing help with a crack addiction, knowing that’s inevitably going to leak sooner or later and lead to a flurry of judgment from the public because of the stigma attached to such addictions.
He also might have still been at the point where he was able to fool himself into believing that everything was fine. Addicts don’t always realize they need help — and even if they do realize they need help, that doesn’t always mean they’re willing to seek help.