It doesn't matter whether Mignini actually believes in Satan. He believed the murder was ritualistic and was supposed to happen on Halloween but was delayed 24 hours due to a dinner party. He then composed an entire theory around this. There is nothing more to discuss on this point.
There is no comparison between the MoF murders and the Kercher murder. The Kercher murder is consistent with millions of murders that occur around the world on a daily basis. It was bloody and it was violent, but there was no reason to think it was anything more than a single act performed by a single attacker. There was absolutely no basis for connecting it to some Satanic, ritualistic murder. That was all in Mignini's head.
Don't talk rot. For an occultist, Halloween lasts all the way through 1 November. The Christian tradition follows the Jewish one in that Holy days commence the evening before, hence almost all holy events consist of two days, not one evening. (For example, Christmas Eve and Christmas Day.)
Please don't say occultists do not exist. I know at least three sincere Wiccans. They gently joke about how they get to celebrate the Winter Solstice six days before we get to pig out over Christmas.
I have known a couple of people take the dark arts very seriously, to the extent of prancing around Highgate Cemetery, at night, and avoiding nuns, as seeing one freaked them out, etc. This fear was real, as they took the type of drugs that caused paranoid delusions.
Heck, the Beatles had Aleister Crowley on the cover of their 'Sergeant Pepper's Lonely Hearts Club Band'.
It wouldn't surprise me if someone dabbling in the dark side of Halloween out of their heads on hallucinogens and seething with teenage rage and jealousy did think it a good idea at the time. Both Knox and Sollecito referenced the remains of Vampire blood make up on Mez' chin that afternoon when she got up late. Both Rudy and Raff were fixated on Vampires and drinking blood.
Why wouldn't detectives moot the fact it happened at Halloween could be a factor the murder/rape, bearing in mind there was no burglary attempt.
Fact is, at the remand trial, Mignini was obliged by duty to put forward a possible motive for the crime. This is only the prosecutor's best guess at that early stage. Ultimately, as the investigation develops, the theory evolves, and in any case, it is the court which determines the motive, not the prosecutor.