An often-raised question concerning Final Fantasy VII is "Why did the WEAPONs behave in the manner that they did?" This is asked due to them having been created to protect the Planet, yet we typically only ever find them attacking the Homo Sapien populace, whereas Meteor is the threat at hand. Further, it is JENOVA/Sephiroth who would likely cause the death of the Planet once Meteor had struck by asborbing the Spirit Energy that the Planet would send to heal the wound rendered by Meteor. One major point of curiosity is why the WEAPONs make no attempt to penetrate the barrier around the Northern Crater (or blast their way under it) and seek to attack JENOVA/Sephiroth. Here, I will attempt to answer these questions and explain the behavior of the WEAPONs.
Quite simply, the WEAPONs were an automated defense system that the Planet had originally created to defend itself from the Crisis from the Sky, JENOVA. However, due to a small band of Cetra confronting and confining the Crisis, it was no longer necessary to use the WEAPONs and they were placed into a state of hibernation, to awaken again should the Planet face a crisis from JENOVA or another threat:
(From the Original Crisis Report tapes at Icicle Inn)
Professor Gast
"Ifalna, can you comment on the thing called 'Weapon'?"
Ifalna
"Yes, Professor."
"The one the Professor mistook for a Cetra... was named Jenova.
That is the 'crisis from the sky'."
"The Planet knew it had to destry the 'crisis from the sky'..."
"You see, as long as Jenova exists, the Planet will never be able to fully heal itself."
Professor Gast
"Back then, Weapon was a weapon the Planet produced of its own will?"
Ifalna
"Yes, but... There is no record of Weapon ever being used."
"A small number of the surviving Cetra defeated Jenova, and confined it."
"The Planet produced Weapon... But it was no longer necessary to use it."
Professor Gast
"So, Weapon no longer exists on the Planet?"
Ifalna
"Weapon cannot vanish. ...It remains asleep somewhere on the Planet."
"Even though Jenova is confined, it could come back to life at some time..."
"The Planet has not fully healed itself yet. It is still watching Jenova."
Once placed into hibernation, they would only activate during times in which the Planet's crisis level was high enough that the Planet itself might be in danger. Once activated, however, rather than seeking out the cause of the threat to the Planet and targeting it, they would destroy
everything. Not just anything that might pose a potential threat to the Planet, as is often believed to be the case, but everything that was alive. They had no programming pertaining to the effect of "Seek out the cause of the planetary crisis and eliminate it." The programming was simply "Annihilate anything that's alive." We know this because of what information Hojo reveals concerning the WEAPONs at the Northern Crater before Meteor is summoned:
Hojo
"Weapon..."
"Then it really does exist... I didn't believe in it."
Rufus
"What does this mean?"
Hojo
"...Weapon. Monsters created by the Planet."
"It appears when the Planet is in danger, reducing everything to nothingness."
"That's what was stated in Professor Gast's report."
We are further able to ascertain this conclusion due to behavior of the WEAPONs that is mostly never shown, but is indicated either through dialogue or through us witnessing their visual results:
(When Tifa wakes up in Junon, a week after Meteor has been summoned.)
Tifa
"How long... was I asleep?"
Barret
"Lessee now...... Must've been about 7 days."
Tifa
"What about Sephiroth?"
Barret
"You ain't over it yet. Remember that huge light, in the Northern Cave?"
Barret
"Since then, the crater's been surrounded by a huge barrier of light."
"Everyone knows Sephiroth sleeps in that big hole, protected by the barrier."
"We can't do a damn thing about it. We just gotta wait till he wakes up."
Barret
"And on topa that, some huge monster called Weapon's been on a rampage."
Tifa
"...Weapon?"
Barret
"Remember that huge monster that was made with Sephiroth, at the bottom of the crater?"
"Well it's up here now. They say it's some legendary monster from the past."
Tifa
"Weapon... is protecting Sephiroth?"
Barret
"Dunno. But he's up here goin' around tearin' [img]/xxx.gif[/img][img]/xxx.gif[/img][img]/xxx.gif[/img][img]/xxx.gif[/img] up."
From this, we can ascertain that Hojo's information concerning WEAPON is correct, as Sapphire WEAPON, at the least, is going about attacking randomly ("...he's up here goin' around tearin' [img]/xxx.gif[/img][img]/xxx.gif[/img][img]/xxx.gif[/img][img]/xxx.gif[/img] up"). That the WEAPON had specific targets is not implied.
Something else to consider is Ultimate WEAPON's behavior, of which we can see both the results after the fact in one case, and the results as they occur in another. First to note is that once Meteor has been summoned and AVALANCHE acquires the Highwind, the area in front of Junon now features a giant crater into which the sea has flooded due to the Planet's surface in the area having been ravaged all the way out to the ocean. There was once a forest (full of life; full of Spirit Energy; take note of this for a matter to be brought up soon) in this location that has since been utterly destroyed, and Ultima WEAPON is found on Disc 3 of the game hovering over this crater, indicating that the WEAPON is what caused the damage to the Junon area:
Ultimate WEAPON near Junon.
Something of further consideration is that Ultimate WEAPON made an assault on Mideel, an area of the world in which its residents lived relatively in harmony with nature. Granted, they would have used trees to create their villages' structures, but for the most part, they were posing no threat to the Planet. There wasn't even a Mako Reactor on the entire continent on which Mideel resides, and the closest one would have been on the eastern continent at Fort Condor, with that Mako Reactor having long since fallen into disservice. In other words, the people of Mideel were no threat to the Planet and there must have been another reason for attacking them.
Here we come to the matter I previously mentioned in regard to the forest that Ultimate WEAPON destroyed: The link between both of these areas that posed no threat to the Planet were that they both contained Spirit Energy in the living beings present.
We already know based on what Hojo said concernin Gast's report and the behavior we see demonstrated by the WEAPONs that they will reduce "everything to nothingness" indiscriminantly, except in regard to differentiating between living targets/areas populated with life, and non-living targets/areas not populated with life. The WEAPONs' targets are creatures that contain Spirit Energy.
As for why this may be so, the WEAPONs only arise when the Planet itself is facing a crisis. During such a time, what would likely be of greatest importance to the Planet overall is its own survival. Thus, in having the WEAPONs annihilate all living things, they would be causing Spirit Energy to return to the Planet and add more strength to the Lifestream. For evidence of such a concept, we can look to Final Fantasy VII: Before Crisis, in which Fuhito of AVALANCHE -- attempting to assassinate President Shinra -- tells the despotic corporation's head that his death will be of benefit the Lifestream:
Fuhito
Heh heh. So we meet again, President Shinra. There’s no need to fear. Your death will release your soul into the Lifestream and help this planet.
(Note: For a translation of Final Fantasy VII: Before Crisis' script, refer to this website:
Gunshot Romance.)
Based on all of this, we can conclude that the behavior of the WEAPONs was not to target the source of the threat to the Planet, but to annihilate all life, strengthening the Lifestream.