Hussein, 19, stabbed sisters Bibaa Henry, 46, and Nicole Smallman, 27, to death in a Wembley park after making a blood pact with a demon.
Since his Old Bailey trial, it has emerged that he was an active member of online forum Becoming A Living God, set up by black magician E A Koetting.
Parts of Hussein’s pledge to “sacrifice” women for power and wealth appeared to have come from the US author’s work.
Yet Koetting continued to promote his books to more than 200,000 followers on Facebook and YouTube.
He has 87,000 YouTube subscribers and 128,000 on Facebook. If 0.1% of people take that seriously, as Danyal Hussein clearly did, and think this is what I have to do to become famous, that's 200 potential murderers
Professor Matthew Feldman, director of C4ARR
Following PA’s investigation with the Centre for Analysis of the Radical Right (C4ARR), Facebook said it had removed Koetting’s page and Instagram account for violating its dangerous individuals and organisations policies.
On Wednesday, YouTube issued a statement to say it was “reviewing” the content.
The company stated: “Hate has no place on YouTube, and we are deeply saddened by this terrible incident.
“We have strict policies to ensure that our platform is not used to incite violence and we are in the process of carefully reviewing the content against these stringent rules.”
Professor Matthew Feldman, director of C4ARR, said some of Koetting’s work could amount to incitement to murder.
He said: “This is the best example I have come across of someone saying this is what you must do to become strong, powerful, rich.
“He has 87,000 YouTube subscribers and 128,000 on Facebook.”
“If 0.1% of people take that seriously, as Danyal Hussein clearly did, and think this is what I have to do to become famous, that’s 200 potential murderers.”
In his blood pact to King Lucifuge Rofocale, Hussein had pledged to “sacrifice” six women every six months to win a lottery jackpot.
Professor Feldman said Koetting had written about blood sacrifices to become rich, attractive and powerful – and even named the same demon, Lucifuge Rofocale.
His texts have also highlighted motiveless killing and making a blood pact.
Professor Feldman went on: “Koetting’s written works include texts that openly discuss and encourage murder.
“One book, Works Of Darkness, describes how to murder another person with a knife in a ritual sacrifice.
“Another book, Apex Of Eternity, advises people to study the terrorist handbook, provides practical guidance on how to kill another person.”
It quotes child murderer Ian Brady saying: “Always remember the first rule of murder: never kill a person that you have a reason to kill.”
One passage says: “What we’re looking for is the knowledge and skill needed to kill with any weapon, with no weapon and from a distance (as with explosives or traps).”
Professor Feldman said: “The text, in particular paragraphs and taken as a whole, can act as an incitement to murder.”
Apex Of Eternity was written for a Nazi Satanist organisation called Tempel ov Blood (sic), which has been cited as a major influence in seven recent UK terrorism prosecutions, six of which involved teenagers.
Tempel ov Blood is said to be the US branch of the UK-based Nazi Satanist group Order of Nine Angles (O9A).
In an apparent reference to Tempel ov Blood, Koetting wrote in one of his books that he joined “an American cell of the notorious British Order of Nine Angles”.
In a YouTube video, which has been viewed more than 17,000 times, Koetting discusses human sacrifice.
He says: “When you destroy the victim there is a release, a massive explosion of power and energy.
“If you can harness that and push it towards a goal, it’s powerful beyond most other forms of magic. It’s the blackest magic without a doubt.”
Later, he adds a caveat that he does not advocate harming anyone “to cover myself legally”.
Koetting follows a malignant form of Satanism called the Left-Hand Path, which grew out of the non-violent Right-Hand Path, which generally promotes white magic like ouija boards.
This summer also saw another double murder in Russia allegedly linked to a Satanist sect.
Professor Feldman suggested more could be done to support moderators of online content, and social media firms should “deep dive” into the background of figures like Koetting when they reach 10,000 followers.
Hope Not Hate, which works to combat far-right extremism, has previously called for O9A to be proscribed by Government.
In April, Hope Not Hate chief executive Nick Lowles said: “Since we started to warn about O9A’s growing influence, a serving US soldier has been indicted for planning a terror attack with O9A members, and the Canadian government has moved against the group, too.”
Patrik Hermansson, a researcher for Hope Not Hate, told PA: “I do not think O9A propaganda should be available on mainstream media platforms. We would appeal for bans on that.”
But he added: “Just limiting access to the material is not going to be enough.
“We think these things are fundamentally encouraging terrorism so we have to go after the organisations themselves.”
Ludo Orlando, co-founder of Reclaim These Streets, said there was an “undeniable link between far-right ideology and misogyny”.
She said: “We live in a society where sexual assault is considered a ‘prominent theme’ among far-right channels and we are surrounded by a culture that not only accepts violence against women but endorses it.
“These dangerous ideas and the feelings of entitlement and ownership of women are dangerous and must be stopped instead of being advertised and broadcast on sites like Facebook or YouTube.
“We need all the possible tools to protect women’s lives, including making misogyny into a hate crime to enable prosecution.”
The PA news agency has contacted E A Koetting for comment.