No time to hunt up the page right now, but I'm pretty sure that before Tuuri was even bitten Lalli saw major trouble coming. There was a page when he said they should head home, right then, and instead the crew decided to explore one more location first; and then one where he said more or less 'we should have gone home; but it's too late now'.
He presumably didn't know exactly what was going to happen, or he would have gotten that troll, or at least tried to, before it bit Tuuri. But he's known for quite a while that disaster of some sort was going to happen.
What we don't know is how accurate his form of knowledge is. Maybe it's not 100% and there's still some chance that things will work out.
-- on the other subject, even in far less drastic cases than the one I cited, it can be infuriating rather than soothing to be told that things will be OK when they clearly won't be, or when it's unlikely that they will be. It is indeed something people commonly do, intending to be comforting; but, unless it almost certainly will be all right, and/or the comforter can do something about it, I'd avoid it. 'It'll be OK, I'll drive you to work so you won't lose your job because your car broke down' is great. 'It'll be OK, you'll get over your grief about your partner's upcoming long painful death from something incurable' is fairly awful. Even in some cases in which it almost certainly will be OK it's likely not to be a good thing to say; 'It'll be OK, when you're 35 you'll be glad that person broke up with you back when you were 15' is not particularly useful; while some people may take it well others may be furious.
And it's all too likely to come across as Róisín said is true in some cases: as wanting the person in trouble to join in denial so that the person supposedly comforting them can feel better. This may not be what's meant; but it may well be what's heard.
TL; DR. Short version: when trying to comfort someone, please remember that people are different, and that to be effective comfort needs to suit the one being comforted.
Slightly longer: providing information about those differences is not meant to be an insult; it's meant to be an explanation. If you and/or your friends been going around telling everybody 'it's going to be OK', this doesn't mean those of us who don't find that helpful think you and/or your friends are terrible people. It only means that we'd like you to think about those differences in the future.