The theory "Mikkel disturbed their resting place and thats why ghosts started to chase him" just didn't feel right. Sleipnope seems to be rational enough to understood that all this intrusion was just incidental and no harm was meant. And, Mikkel wasn't even the first target of their attack; first was Sigrun, who never ever were near fort, and ghosts never actually meet her.
You're missing a couple of details here, though. First of all, the fact that the ghosts left their resting place and started chasing the crew well
before they merged into Sleipnope. That happened... well, the earliest thing that could rationally be called an incarnation of Sleipnope was created either during or immediately after the first attack, when Reynir was trying to fend them off and then when Onni swooped in to get them off the crew's backs. Prior to that, the ghosts that Reynir saw creeping up on the tank were a group of individuals, not any sort of legion.
As for Mikkel disturbing their resting place... it does fit with the fact that the animal tracks were all avoiding the place (
they knew something bad would happen to them if they went in there). I don't think at that point the ghosts had any sort of collective consciousness - they were just following the tracks of the person who'd disturbed them. We also have it from Minna that who was and wasn't the first target of their attack was a matter of pure luck; Mikkel and Sigrun went down first (and
both of them at the same time) because they were in the shadows, where they were vulnerable, whereas Emil and Tuuri were lucky enough to have been in the last remaining patches of sunlight right at that moment, and Reyir was saved by his latent mage powers kicking in.
So, there must be some deeper, more important reason, why a bunch of ghosts started to follow the expedition. It took a lot of efforts for them, after all; after failed attack, Sleipnope looked pretty miserable... actually looked harmed and suffering.
So, we have the rational entity, that considered destruction of expedition important enough to take the risks and suffer for that. And the Sleipnope taunts to Lalli/Reynir seems to be... aimed.
My personal theory is that this chase didn't start out as any deeper more important reason - it was just a bunch of ghosts reacting to someone who'd come into their territory. But I do think it likely that it
became a deeper more important reason as time went on and Sleipnope continued to evolve. Onni has stated outright before that ghosts that find themselves unable to move on have a tendency to grow angry, and these
particular ghosts could also have been harboring some latent anger over the failed cure and the fact that their caretakers lied to them. They wouldn't necessarily remember what the anger was
about, but it would still be there nonetheless.
So then, something living and oblivious wanders into their domain. They're no longer capable of rational thought at this point or of parsing who's
actually to blame; the only thing they know is that a living target has ever so conveniently presented itself. So, they follow it. And it leads them to still
more convenient targets.
Then, the attack happens, and is thwarted... but it has the effect of merging them into a single legion. As such, this legion begins to acquire the first glimmers of something that they hadn't had before: purpose. It doesn't matter whether this purpose will actually help them or not; it doesn't matter how many times they're thwarted. They have an actual goal now, something to work towards in lieu of the moving on which they've become incapable of doing, and so they pursue it singlemindedly. As they move on, they acquire more minds and their purpose grows sharper; they begin not only to plan but to rationalize. If they must suffer, others must suffer with them.
If so, its a pretty... stupid consciousness. Most trolls demonstrate pretty simplistic behavior, not even on the level of mammals. And giants seems to be evless intelligent.
Considering Cthulhund and train-giant head... in both cases brain seems to be relatively intact. With Cthulhund, the remission period was at least several hours long, but actually Cthulhund wasn't very agressive in the beginning. With the head of train-giant, we seems to have at least some partially intelligent reaction, but the observation period was too short to make a conclusion
That's the issue of thinking of Rash creatures as animals. The thing is, though, Rash creatures
aren't animals as we know them. The Rashed are a disease. And what's the singular purpose of a disease? To spread. Thinking from the viewpoint of a pathogen, it's perfectly rational to sacrifice its current host body if it can in turn infect
another host which can then go on to spread the disease (and considering that the Rash is at its most virulent in the early stages... there would be plenty of advantage in infecting a new host body as opposed to preserving the old one). There are many,
many examples of animals that have been infected with some virus or fungus or parasite acting in ways that are counter to their own survival: ants climbing the tallest blade of grass so they'll get eaten by a cow, mice losing their fear of cats... preservation of the host body isn't important, but propagation of the disease
is.