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[Star Trek] [Voyager] Capt. Janeway's decision to ally with the Borg against Species 8472

I'm wary of asking this, since it skirts the borders of objective fact(in the Star Trek Universe) but here goes:

TLDR: My core question is this: was Capt Janeway sufficiently justified in believing that Species 8472 were a comparable threat to all galactic life as the Borg? After a single violent encounter, with a single individual of the species, she deigns them a greater threat than the entire Borg Collective. And thus justifies an alliance with Borg, where she helps them fight off Species 8472. This seems at odds with various Starfleet norms, not least being the Prime Directive. Am I missing something or disregarding some facet here?

I've been rewatching Star Trek: Voyager recently and I've just finished the episode VOY 4x26: Hope and Fear. In that episode, >!the last survivor of an alien race - recently obliterated by the Borg - attempts to trick Voyager's crew and sacrifice them to the Borg. He does this in revenge for Voyager helping the Borg overcome Species 8472 - a conflict they would have otherwise likely lost.!< One particular line was striking - "Who were you to make that choice? A stranger to this Quandrant!", and it echoed a like from Chakotay one season ago (in VOY 3x26: Scorpion) , where he says - "How much is our safety worth? We'd be helping a species guilty of killing billions". At the time and later Janeway points to what Species 8472 did to her crewmember Harry Kim and the psychic visions transmitted to another crewmember Kes warning "The weak will perish".

But this seems a rash judgement in my opinion. I assume 8472 are new to our universe and their initial contact, possibly sole contact before the crew of Voyager, was with the Borg - an antagonistic species if ever there was one. Voyager's crew encounters one of their pilots in a derelict Borg cube, in the wake of a large battle, where he attacks Harry Kim and infects him with a unique "virus". But this doesn't seems to have been a deliberate act, but rather a feature of their biology. Their subsequent interactions, including psychic threats, are after Voyager has allied with the Borg. Looking at these actions as those of soldiers and field commanders in a warzone, it seems a stretch to characterize the whole species. Imagine if the Klignon's were judged solely by their warcrys and battle prowess?

Furthermore, consider how Starfleet has interacted with other inscrutable, violent forms of alien life. For example, the Crystalline entity in TNG - a creature with a far higher Federation body count than 8472, but they still attempted to establish communication with it rather than kill it.

Finally, I would consider the Prime Directive applies here. Since Species 8472 has made no overt threats towards the Federation or Voyager, this seem like a purely internal matter between them and the Borg. Simply because the two species involved are as advanced(arguably more so) than the Federation, does not preclude the non-inteference that is the foundation of Star Fleet. To interfere is to change the natural course of history for both species, and in fact for all species in the Delta Quadrant.

Thus I submit that it seems to me to have been the wrong choice going by Federation principles and indeed the general principle espoused by Star Trek as a whole.
gpt
The facts available to Janeway after the initial encounter with 8472 but before entering negotiations with the Collective were:

* 8472 is advanced enough to be a major threat to the Federations most powerful adversary.
* 8472 have shown themselves to be hostile when they attacked a Starfleet officer.
* 8472 have fired upon a retreating Federation starship without provocation.
* 8472 Have threatened to purge the galaxy (i.e. kill everything).
* Borg generally don't kill but rather assimilate.
* Borg assimilation is reversible (Locutus/Picard).

It is perfectly reasonable to conclude that 8472 is a more pressing threat than the Borg.

The PD certainly doesn't apply, it ceased to be an internal matter when 8472 fired on Voyager.