The Serpent tells Skadi he knows Thor is destined to kill him; what he's doing is all for her, so that she could rule after he's dead. But then, when Thor does indeed kill him, the enchantment that made Skadi and other henchmen of The Serpent possess superheroes and villains immediately dissolves, apparently banishing Skadi and the others to whatever place they came from in the first place. So if The Serpent knew he was gonna die, what exactly was his plan? As such, his actions didn't benefit Skadi at all.
What the heck is up with Odin's behaviour in this series? He forbids Thor to try to protect Earth for The Serpent, going as far as attacking, humiliating and imprisoning him (and he should know this is only gonna make Thor more likely to go against his will), instead of, you know, explaining the prophecy to Thor, so he would know why Odin doesn't want him to stay on Earth. Also, Odin is willing to sacrifice all the inhabitants of Midgard without any second thoughts, speaks of them as if they are insects or something, and doesn't even consider any alternative ways of defeating The Serpent that would save humankind. Previously Odin has been depicted as a good father and wise god, and one that cares about Midgard and humanity, but in Fear Itself he acts like a petulant despot.
The answer for both is sadly the same. Fear Itself is a poorly written story. Fraction takes the vices of characters and runs with them without balancing out any of the virtues. Odin should have had a better plan with razing the Earth as a back up. That fails since Fraction was having Odin attacking Earth at the heart of the Serpent's army. All in all it is a badly written story.
Why didn't Beta Ray Bill appear? This isn't a complaint about missing out on a favorite character, either—Bill is at least as strong as Thor (if not technically stronger since he could beat Thor without Stormbreaker) and is his oath-brother. He should have at least been called in during Odin's attempted razing of Earth. I know he was appearing in Annihilators at the time, but shouldn't Fear Itself have overridden that? Plus, there was the potential for another Thor/Bill match-up.
Also, the rewriting of Marvel's Norse mythology to accommodate the Serpent is tenuous at best.
We know Odin had two brothers—Vili and Ve, who sacrificed themselves to merge with Odin into a super-being to defeat Surtur. (V1 #349, the kick-off of "Ragnarok'n'Roll, a.k.a. perhaps the greatest Thor story ever told and one of the major climaxes of Simonson's run.) I guess the Serpent was imprisoned before the combination, and I get that he was written out of history, but there's a lack of information about how he fits in. Was the Serpent Bor's son, or Bestla's, or both?
Thematically, it would make a lot of sense if he was Jörmungandr, as he is about to destroy the world, and he and Thor are destined to kill each other, just like in the original Norse myths. But why make him a brother of Odin instead of the son of Loki then? Did they think wouldn't appear to be a big enough threat if he was "only" Thor's nephew (though that didn't stop Simonson from using him correctly), as if the readers couldn't comprehend that these are immortal gods, and even a nephew is ancient by human standards? They would've even had a perfectly good explanation for bringing him back, even though he was killed in Thor #380, since the previous Ragnarök cycle came to an end in the final issue of Thor Volume 1, and a new cycle started with the new volume, with all the dead gods coming back to life.
Loki was a kid at the time, so having his son be the villain would be a bit odd... but on the other hand, Thor (both the mythological figure and the superheroic version) has encountered much weirder things.
This is a bit of an art fail: when does he turn into a gigantic monster? He gains a ton of fear-power, then a few pages go by, and BOOM he's a giant snake. We couldn't get a few panels of him transforming? It's kind of a key moment.
You see him transform in Journey into Mystery #629.
Why did the Serpent suddenly grow a beard for a while before he switched to his younger black-haired (and beardless) appearance? It's not Depending on the Artist since it's all drawn by Immonen and not Off Model since it appears in more than one panel. Except for those panels he only ever has a mustache.