Exactly when Malekith became in on it is a really interesting point. You can read it as him being in concert with Morathi from the very start, to him being swayed at some point along the way, possibly by the Circlet of Iron, to him only crossing the line that very last day at the Shrine of Asuryan.
It's a bit odd for the narrative to decry the pleasure cults as being the cause of the problem when Morathi is, well, Morathi, but I suppose they can only shift the narrative so far away from the truth.
Isn't it common knowledge that Morathi was behind those cults? Oh and the princes sabotaging the flame of their god is pure copium.
Malekith publicly denounced and arrested Morathi and her coterie for being members of it, which is why he was trusted with the authority to further pursue and eradicate the Cults.