Right. So. First time through reading American Gods, unless you've had some kind of forewarning or are extremely extremely observant, especially in regards to fine details--Horus' appearance and subsequent Sudden Godmode (ahaha) is TOTALLY OUT OF THE BLUE.
However, my opinion is that there's stuff that suggests otherwise--that, like Loki and Wednesday's orchestrated war of gods, everything that went wrong with that scheme was orchestrated as well.
From the Cairo bit onwards, there's random passing mentions of hawks being spotted every now and then, which suggests, since Neil Gaiman is a devious little minx, that Horus has been tailing Shadow. Sure enough, when Shadow meets Horus in person for the first time during the vigil, Horus says a whole lot of random shit, but also something that pretty much confirms this: "I know you. I watched you in Cairo. I watched you after. My sister likes you." Because Horus is a random crazy, one is most likely inclined to dismiss the third of these sentences as unrelated, and assume that he's been following Shadow just because he's weird and felt like it. But back during the Audrey Burton incident, Bast tells Shadow, "I got eyes wherever my folk walk." And again, after Shadow dies--"my people have been keeping their eyes on you, for me."
And there's another thing--Horus notices Shadow is on the verge of death, but all he does is say, "you are dying. Aren't you?" before flying off. And then Shadow dies--and meets Zorya Polunochnaya (on the topic of whose involvement I reserve judgment for now, because I don't trust Gaiman and haven't gone over the Russian bits in as much detail as the Egypt bits yet) before meeting Bast--who he allows to chose a path for him. After that, it's the Egyptian crew who takes care of his after-death niceties--in particular, Thoth tells him, "I requested that I be the one to come for you", doesn't give Shadow a straight answer when he asks why, and then says "It doesn't matter that you didn't believe in us. We believed in you." And then the weighing happens, after which Bast tells Shadow, "I had hoped that you would do some good, in the current troubles." (And in the subsequent pre-battle scene, one notes the absence of any Egyptian gods. Or the Russian crew, come to that--but like I said, I need more information to figure out that bit.)
And then Horus goes to find Easter so they can revive Shadow--"If he is gone forever, it is all over." And Shadow figures it out and fixes everything and it's all over etc.
Now, I still need to do another reread to figure out some things--such as the involvement of the Russians and the Americans, where Mr. Nancy actually stood on all this, etc.--but still. I think? There's something going on here, and there's more to be said, plot-wise, at least on the matter of the Egyptians. I think Bast is up to something. I think Horus and Thoth are in on it, and probably Anubis, too. I think the Russians might be involved somehow, and possibly the Norns, and maybe Johnny Appleseed and Wisakedjak, too. But this is something I like about Neil Gaiman--at the end, there is the big reveal of This Is What Happened, but there's also a bunch of other stuff that isn't made entirely explicit either, and a lot of it seems pretty damn important, but you have to go through and pick up the bits to piece it together anyway.
At any rate, this is all basically just a sketch of the relevant issues, but I think, in light of all this, Horus' actions seem a bit less random and inconsistent, no?