Almost everything written about Crowley has to contend with the fact that he was an unreliable narrator about himself, and deliberately so. He courted the tabloid press, accepted the label of the wickedest man in the world with evident satisfaction, and wrote an autobiography he subtitled an autohagiography. He also exaggerated, invented and suppressed. The result is that the most detailed source for his life is the one least safe to use, and biographers still disagree about episodes for which he is the only witness.
The reputation was built from real material and inflated packaging in roughly equal measure. He was a serious mountaineer, involved in early attempts on K2 and Kangchenjunga. He was a heroin addict, the drug having first been prescribed for asthma. His sexual life was openly bisexual and promiscuous at a time when that alone was ruinous. He was a competent poet and an intermittently brilliant prose stylist. And people close to him suffered badly, including partners who died or broke down. The death of Raoul Loveday at Cefalu in 1923 was consistent with an enteric infection from local water, but the press treatment that followed, driven by Loveday's widow, produced the version that stuck.
Two claims are worth separating from the rest because they are commonly repeated and thinly supported. That he was a British intelligence agent rests mainly on his own account of his German-language propaganda work in New York during the First World War, and historians differ on how much of it was sanctioned by anyone. That he performed human sacrifice derives from a passage in Magick in Theory and Practice which his own later notes make clear was a coded reference to semen.
He also mattered, which is the part most easily lost. Rock music, science fiction, modern Wicca and the working vocabulary of contemporary occultism all carry his fingerprints, and the figure on the Sgt. Pepper sleeve is the same man the popular press had spent decades treating as a monster. Assessing Thelema through Crowley's personality is unavoidable, but it tends to substitute for assessing what he actually wrote.