The administration of colonial Sierra Leone believed that the Leopard Society was real and instituted provisions to combat it:
Then followed provisions making it penal for any person without lawful excuse to have in his possession or keeping any of the articles mentioned in the Schedule, viz. :
" (a) A leopard skin shaped so as to make a man wearing it resemble a leopard ;
" (6) A three-pronged knife ; and
" (c) A native medicine known as ' Borfima ' " ;
and under the Ordinance the police were given powers where there was reasonable ground of suspicion to arrest and to search without a warrant, and heavy penalties were imposed for obstructing the police.
Normally, I would be skeptical of the claims that such cult has existed as it sounds like coming straight from pulp fiction. Yet considering the existing records and, as it happened, among other places, in Liberia, the home to glorious General Butt Naked (a mass murderer turned preacher who practiced ritualized cannibalism and child sacrifice), I'll give the story the benefit of the doubt.
The degree to which Mende and Sherbro people in colonial Sierra Leone 1) engaged in 'human leopard' murders and/or 2) believed that other people were engaging in such murders is the subject of scholarly debate. There is not a universally accepted answer. Some scholars, such as Christine Whyte, approach the phenomenon as a type of moral panic, produced by British stereotypes about African cannibalism (Whyte, 'The Strangest Problem: Daniel Wilberforce, the Human Leopard Murders, and the Special Court in Sierra Leone,' in Harald Fisher-Tiné, ed., Anxieties, Fear and Panic in Colonial Settings, Palgrave-Macmillan 2016). Others, such as Marianne Fermé, have concluded that the episode drew on genuinely extant beliefs about the relationship between power and cannibalism/witchcraft in precolonial Sierra Leone, and that while we can't know for sure whether such murders were committed, it is at least plausible, and that Sherbro and Mende people did believe that human-leopard murders were being committed (Fermé, The Underneath of Things, UC Press, 2001).
I tend to think that the latter view is correct, at least to the extent that Mende and Sherbro people believed that human leopard murders were a real threat - if it was a moral panic, it was one that was shared between colonizers and colonized. However, the latter group almost certainly strategically used accusations of human leopard murder as a way to depose unpopular chiefs and otherwise settle political scores (Fermé has some on this, as does Paul Richards, "Chimpanzees as Political Animals in Sierra Leone," in John Knight, ed., Natural Enemies, Routledge, 2000).
For a study of similar phenomena elsewhere on the continent, I strongly recommend David Pratten, The Man-Leopard Murders: History and Society in Colonial Nigeria (Indiana University Press, 2007).