Humans have reason to hate themselves even more than they hate the Vegans. The Vegans are basically tourists, lingering among the ruins of the last non-radioactive territories on Earth. The historical Three Days of War occurred between human and human, or rather between dirty bomb and dirty bomb. The back cover of "This Immortal" ironically states "Welcome to Earth (Pop. 60,000)." (Later in the text, we learn that Earth's population is more like four million).
"This Immortal" (1966) was Zelazny's first SF novel, and it shared the Hugo Award for Best Science Fiction Novel of the Year with Frank Herbert's "Dune," so it is no lightweight post-Apocalypse adventure story. In fact, I think the best way to understand "This Immortal" is to read Lawrence Durrell's chronicles of the Greek Isles, most especially "Prospero's Cell" and "Reflections on a Marine Venus"---or better yet, read Percy Shelley's "Prometheus Unbound," which is referred to more than once in this novel.
Like Shelley's Prometheus, Zelazny's hero, Conrad Nimikos is mankind's potential savior. Like Prometheus, he suffers and almost self-destructs in trying to save his chosen people (the humans who remained on Earth). Instead of stealing fire from the gods, he sets out to steal back Earth from the Vegans.
At an earlier stage in his career (nobody knows quite how old he might be), Nimikos was a terrorist. Now he has lost some of his destructive impulse (his hubris), and sets out to protect the Vegan, Cort Myshtigo in order to discover the alien's true purpose in touring Earth's remaining monuments.
The real mystery of "This Immortal" is not so much the Vegans' intentions toward Earth as it is the true identity of Conrad Nomikos. His mistress, Cassandra playfully refers to him as a 'kallikanzaros' (this is where I had to refer to my Durrell), which a Greek term for a little cloven-hooved satyr, who causes mischief of every kind. Conrad is a huge man, superhumanly strong, but he is also incredibly ugly and walks with a limp.
Did he at one time have a cloven hoof?
Like the kallikanzaros, Conrad is a trickster, and deceives Vegans and humans alike into thinking his previous incarnation, the terrorist leader 'Karaghiosis,' has died in a boating accident.
Once more turning to Durrell, we learn that 'Karaghiosis' is a stock character in Greek drama--in fact, "the embodiment of Greek character...based on the idea of the impoverished and downtrodden little man getting the better of the world around him by sheer cunning. Add to this the salt of a self-deprecating humour and you have the immortal Greek."
So Nomikos-Karaghiosis-Prometheus is the embodiment of 'the immortal Greek,' who might or might not be--according to a hint at novel's end, plus references to the kallikanzaros--the Great God, Pan.
Conrad himself refers to "Prometheus Unbound" as "Percy B's dud epic," but then, all of Zelazny's heroes tend to be self-deprecating.
Zelazny has succeeded in capturing the spirit of a people and place in "This Immortal." I think his Hugo was well-deserved.