I know that "suicidal doomsday cults also existed long before the Internet,"
but they were different from the Heaven's Gate.
Jim Jones started out as a Christian-Socialist-Humanist Utopianist, and only turned 1) authoritarian, 2) paranoid, and then 3) homi-suicidal when it became increasingly obvious that his Utopianism was actually Utopian, i.e. a delusion, and that the authorities were on to him.
The weirdness of the Heaven's Gate with their cult of castration and "the level beyond human"
was much more obviously weird from the outset.
I don't think that there's any reason to assume that Bernd Jürgen Armando Brandes would have been a serial killer. Maybe an auto-mutilator ...