Prince Argrath is one of the four big movers driving the Hero Wars forward. To get a handle on him, it is worth looking at the very first presentation of Prince Argrath – in what was the very first published appearance of Glorantha as a setting, White Bear & Red Moon:
These two passages are the basis of defining the Hero Argrath. One more is worth mentioning:
There’s our contours for the Hero. Now it leaves everyone plenty of room to interpret – was Argrath a cynical but charismatic manipulator, was he a religious fanatic, was he a footloose adventuring rogue who found himself with great responsibilities, or was he seeking to redeem his ancestors and his people? Or maybe a bit of all of this? Your Argrath will vary, of course.
Regardless of our interpretation of Argrath the mundane individual, we know that he very much embodies Greg’s Hero Archetype for Glorantha, as Greg took his hero Argat (who defeated Gbaji and his empire in a Great War) and placed him in Dragon Pass to fight Gbaji-become-Red Goddess and her empire (Arkat and Gbaji get their own story a few years later with Cults of Terror).
So imagine Alexander the Great tagged up with Heracles, and that’s the Argrath-Harrek team we have going into the Hero Wars.
But imagine an Alexander the Great that spent his youth as Sabatini’s Scaramouche, Zorro, and the Dread Pirate Roberts!
You can make Argrath the villain of your campaign – certainly from the Lunar Empire’s perspective, that is his role (just as Alexander was a villain in Zoroastrian sources, and is both hero and villain in many Roman sources). You can make him the moral exemplar in your campaign – although Greg rarely built paragons of virtue (even Greg’s beloved Arthur has misdeeds).
I personally view Argrath (and the other heroes) as a remarkable and fascinating individual, who did remarkable deeds and co-exist in the realm of myth and legend. But all of Glorantha’s heroes did some terrible deeds – free, Argrath gave Furthest over to his companion Mularik Ironeye and then later arranged his murder. Like Alexander he adopted foreign ways, and even accommodated Lunar cults and deities (such as Yara Aranis). He spoke with dragons and revived (at least in some manner) the Empire of the Wyrms Friends. But he was also loyal to his friends (of which he had many) and was by all accounts an inspiring, clever, and remarkable individual.
Hating the Lunar Empire for what it did to his family is also a perfectly understandable (at least to Orlanthi) vendetta. The Lunars killed most of his kinship group, took what was rightfully there’s, and tried to eliminate his tutelary deity. That he took the war to the empire is, at least from Orlanthi eyes, not only understandable but laudable.