A weapon is a weapon is a weapon. It is designed to kill, as quickly and as efficiently as possible. That is its purpose.
To me, the Flashpak is not an inhumane weapon. Conventional explosives could take off a limb, leaving you slowly bleeding to death, or just puncture the hull, leaving you to suffocate as the air slowly leaks out of your ship. The flashpak kills in seconds. Boiling, shooting, freezing, vaporizing ... they're all the same. Dead is dead. Inhumane is not the weapon, it's how you use it.
Current non-conventional weapons, such as chemical, biological or nuclear weapons, are generally considered to be inhumane because they are weapons of mass destruction and target victims indescriminantly. There is no way to differentiate between a soldier and a civilian. At the moment, killing (large numbers of) civilians is considered unacceptable (by most civilizations anyway. Certain religions seem to think it is a good thing to do to infidels).
In some respects, the gen-select device could be considered a VERY humane weapon - if used correctly. If your planet is overrun by Kilrathi foot-soldiers, simply release an anti-Kilrathi gen-select weapon into the atmosphere. 100% Kilrathi kills, 0% civilian. Perfect. Using it against a Kilrathi planet could be considered inhumane, but is there any difference between that and just lining them all up and shooting them in the head?
The Flash-pak is a nice toy. One FP can take down a carrier, but then, so can 2-3 torpedoes. Since we don't see them in use in Prophecy, I guess we can use several possible excuses, including but not limited to :
1. Versatility. We know the FP has problems in that it is ineffective against the Versuvius' hull. Are there other situations where it proves ineffective? Is it that superior to a good torpedo in all situations?
2. Reliablity. From memory the FP attaches itself to the hull of a vessel, activates, and fries the interior of the ship. Does this equate to a 100% guaranteed kill? Do they ever bounce off? or not activate properly? The Black Lance were testing their new weapons against the Borderworlds. Were the tests successful?
3. Effectiveness. We know the Versuvius hull is impervious to FPs (if you can keep them out of the hanger bay). Are there other defences against the FP if you know it's coming.
4. Delivery. The only vessels I saw launching FPs were Dragons (I always chose to use it
). Do FPs need a special delivery system, designed to launch flashpaks. What is the range of a FP compared to a torpedo?
5. The ever-reliable "just because we don't see it ..." argument. The FP has limitations. Perhaps it is not the type of weapon to find on a carrier on her shakedown voyage (remember the Midway has no Vampires or Devastators at the start either). Perhaps FPs were unsuccessfully used against the bugs in the first few encounters. Perhaps analysis of the Bug ship materials found that FPs would be ineffective, so they were never used.