Yes, the "heterogeneous" cult did the killings, and they were sea fare men "from the Cape Verde Islands", but there were also wide spread reports such as the Eskimos that also believed in the same Gods.
There is also mention of "undying leaders of the cult in the mountains of China", as well as the "two Lascar sailors" who "helped" Johansen, and the Kanakas aboard the Alert. In other words, Lovecraft takes care to make it evident that the cult, though secret, is worldwide in scope.
Now did this narrator follow some map to Cthulhu from notes that he acquired somewhere? Why didn't the cult members raise Cthulhu themselves? and the monster stopped chasing the narrator who made a narrow escape, so the monster was trapped, no?
The narrator -- if by that you mean the narrator of the story as a whole, Francis Wayland Thurston ("Found among the Papers of the Late Francis Wayland Thurston of Boston", as it says in the note on the first page) -- didn't ever see Cthulhu. As he says at the very beginning of the tale, his part in this was "an accidental piecing together of separated things—in this case an old newspaper item and the notes of a dead professor". What he does is take the reader through the process which led him to these various items, and his piecing together of the parts of the puzzle... very much in the manner of a detective tale.
As for why the cult members didn't raise Cthulhu... they were on their way to do this when they encountered the
Emma, which had been blown off course by the storms caused by the rising of R'lyeh. They then attacked Johansen and his comrades to protect the secret, but suffered defeat and death as a result. The crew of the
Emma, taking over the
Alert (as the
Emma had been too severely damaged during the fight to be seaworthy) decided to follow the trail of their erstwhile attackers and find out what they were so zealously guarding. And, as a result, "what an age-old cult had failed to do by design, a band of innocent sailors had done by accident". The reason Cthulhu ceased chasing after Johansen and his single surviving companion? Well, for one thing, Johansen had the good sense to ram Cthulhu with the Alert in an attempt to kill him/it; unfortunately for Johansen, this "monster" was of such an alien nature that this did not kill, but merely disrupted him temporarily, while he reassembled himself in his original form:
But Johansen had not given out yet. Knowing that the Thing could surely overtake the Alert until steam was fully up, he resolved on a desperate chance; and, setting the engine for full speed, ran lightning-like on deck and reversed the wheel. There was a mighty eddying and foaming in the noisome brine, and as the steam mounted higher and higher the brave Norwegian drove his vessel head on against the pursuing jelly which rose above the unclean froth like the stern of a daemon galleon. The awful squid-head with writhing feelers came nearly up to the bowsprit of the sturdy yacht, but Johansen drove on relentlessly. There was a bursting as of an exploding bladder, a slushy nastiness as of a cloven sunfish, a stench as of a thousand opened graves, and a sound that the chronicler could not put on paper. For an instant the ship was befouled by an acrid and blinding green cloud, and then there was only a venomous seething astern; where—God in heaven!—the scattered plasticity of that nameless sky-spawn was nebulously recombining in its hateful original form, whilst its distance widened every second as the Alert gained impetus from its mounting steam.
While it didn't stop Cthulhu permanently, it did slow him down briefly... enough for Johansen to get the steam up in the
Alert and make good his escape. By this time, the distance was so great that it simply wasn't worth the bother for Cthulhu to pursue him. He was simply not important enough.
Another thing that I don't understand is that someone was having dreams and making sculptures, as if the title of the story made sense, "The Call of Cthulhu".
The title of the story does make sense: it is that call, as the stars become right, which sets things in motion by alerting the cult members to prepare for Cthulhu's emergence, while those unknowing of the dweller in R'lyeh, but who are of sensitive mind, are also plagued by the dream-images being broadcast. To some it gives nightmares, to some only vague impressions; others the call drives mad.
Oh yeah. I also remember that the one group of hetrogeneous cult members who were holding a festival deep in the forest was reported to have raised some spectre of some kind, or something was seen, some monster.
These were the Louisiana cultists. And it is by no means certain such a thing was seen. The police officer who claims to have seen it, if you will recall, is described in these terms:
It may have been only imagination and it may have been only echoes which induced one of the men, an excitable Spaniard, to fancy he heard antiphonal responses to the ritual from some far and unillumined spot deeper within the wood of ancient legendry and horror. This man, Joseph D. Galvez, I later met and questioned; and he proved distractingly imaginative. He indeed went so far as to hint of the faint beating of great wings, and of a glimpse of shining eyes and a mountainous white bulk beyond the remotest trees—but I suppose he had been hearing too much native superstition.
However, Lovecraft leaves this one an open question, and he was fond of having such glimpses in his work (cf. "The Thing on the Doorstep", "The Poe-et's Nightmare", etc.).
I wonder why the cult members killed the professor. I'm also somewhat unclear how the narrator found Cthulhu or was he just reading papers the whole time, that makes a big difference.
The cult members were protecting the secrets of their religion. It has been claimed throughout history that various "mystery cults" would use violence to protect their mysteries, as they were sacred and only to be seen or known by select members... not even the main body of the cult were always allowed access to such things, but sometimes only the very highest members of the priesthood. At least, such was common report; the truth of the matter varied, but it was used fictionally by a great many writers... including Thomas Moore, in his "The Epicurean" and
Alciphron, which influenced HPL enough for him to not only mention them in his Supernatural Horror in Literature, but quote from
Alciphron in two of his stories ("Under the Pyramids" and "The Nameless City"). Should you wish to look at these (they are not actually weird in themselves, but they do have some fine weird passages), you can find them listed, with links, in the thread on "Online Sources for Supernatural Horror in Literature".
As for the question about the narrator, I answered that above. Thurston was reading first the professor's notes, then his account of the scientific meeting and Legrasse's tale, then Johansen's manuscript.
BTW everyone is always killing themselves :+(
I assume you mean "dying" or "being killed" (though there are suicides mentioned in the first chapter). That's what happens when you go up against something as big and alien as what Cthulhu represents... and even so, he is
the very least of the entities hinted at....