: GURPS describes them as "once proud human
: warriors" in its history, and in the Myrmidon
: section itself, "Centuries ago, all the might of
: the Dark stood powerless before the might of the
: Light's Myrmidons...". It goes on to say that
: Balor, unable to beat them, swayed them to his side
: with the promise of immortality.
: If Balor couldn't beat them, Connacht probably couldn't
: either. They kicked major ass, apparently. Of course,
: Balor had nothing at that time, while Connacht had the
: entire armies of the Light with him by the end of M3.
Personally, I doubt that the spearmen are Myrmidons (although in TFL, we do often see Myrmidons and Soulless working in tandem). After all, Gridaksma blades are a very specialised weapon, so it seems likely that they were always the Myrmidons' predominant equipment, although perhaps there were some Myrmidons with big double handed warhammers (as the names Battlehammer, Ironhammer and Bonehammer would suggest). Shuriken discs and throwing daggers would also fit in well with the fast, fluid style of warfare that the Myrmidons favour.
Much as a surprise attack on the Berserks would make a great level, Myth3, as far as we know, doesn't cover the time when Connacht becomes Balor, so we almost certainly won't see the betrayal. I personally think it's more likely that the Myrmidons would have grudgingly joined Connacht some time during his rise (they are described as proud, which probably means honourable as well, and, as such, would respect a great warrior like Connacht, although it's questionable whether they'd ever directly place themselves under his command. Independent allies is probably more in line.)
And the Myrms were never evil. If you were offered immortality, wouldn't you seize it? I assume that they serve Balor either because they gave their word that they would, or because he placed them under a geas of some kind. That, and the fact that most of the Myrms we see in TFL are well and truly insane. :)