Why is Legion helping you whilst geth are fighting you?

Question

(It's difficult to know what to hide as a spoiler and what not to, I've done my best to hide what I think counts as spoilers but if it seems inconsistent or non-sensical, let me know.)

As explained by Legion, mostly in Mass Effect 2, the geth are "software", and form a consensus on how they should act.

In Mass Effect 3, you encounter Legion on the Geth Dreadnought. Despite fighting your way through geth, Legion is friendly. After freeing Legion, it helps you escape, even though other geth are trying to kill you.

I don't understand why Legion is pursuing a different course of action to the other geth.

It makes sense in Mass Effect 2 why the geth you fight against would act different to Legion as they left the geth consensus to "follow the Old Machines", so they are now effectively two people, two kinds of geth, two separate consensuses.

But in Mass Effect 3, those "other kind of geth" are either destroyed or rewritten as per Legion's loyalty mission in Mass Effect 2, so there is only one kind of geth at this point. Legion is therefore presumably the same kind of geth as those fighting you, so why would it work against them?

According to the fandom page about Legion (emphasis mine):

it houses 1,183 geth programs, as opposed to the roughly one hundred found in other platforms, enabling it to operate independently and speak.

Is it that Legion is effectively its own consensus, capable of acting separately to the greater geth consensus? Doesn't this contradict the implication from Mass Effect 2 that it is part of the consensus who chose not to follow the "Old Machines"?

Legion even claims that it was a "difficult decision to make" to ally with the "Old Machines" when you meet him in Mass Effect 3, implying that it is part of the geth consensus, but then helps you anyway.

Can someone explain (ideally with evidence from the trilogy, be it quotes from the games or from BioWare staff or whatever) why it makes sense that Legion, who as far as I am aware is part of the one-and-only geth consensus during the events of Mass Effect 3, would act differently to the rest of them?

(I'll point out that, at time of writing this, I haven't yet done the next missions after the Geth Dreadnought mission, so they might provide additional context; it's been a while since I last played through this game...)

Answer 1

The geth consensus isn't imposed from above. It's a collective opinion openly arrived at by debate between the individual geth runtimes, but they are free to disagree with that consensus at any time. It's just rare that they do, because 1) all geth run the same basic programs and have the same basic way of approaching and solving problems and 2) almost all geth have similar experiences of life; most of them are relatively sheltered in what was once quarian space, and they spend almost all of their time with other geth. So while individual geth might weight different factors slightly differently or have specific expertise that changes their opinion, for the most part they're predisposed to agree with each other most of the time.

The geth "heretics" in Mass Effect 1 and 2 are an exception to the first rule, as Legion points out. They have certain variations on their programming that appear benign on their own, but ultimately mean that they choose to follow the Reapers where a mainstream geth runtime, considering the same arguments and the same evidence, would not. (Which is why there's some suspicion from your party that these variations may not be entirely natural.)

Legion's dissent from the consensus support for the Reapers in Mass Effect 3, in contrast, is of the second type. Legion has much more firsthand experience with the Reapers and is aware of how they operate. Even though Legion is in agreement with the consensus on most issues, this experience forces them to dissent and take direct action to stop the Reapers. Any geth could do this - but only Legion has a strong reason to do so.

Answer 2

While you are correct that the geth form a consensus, it is important to understand what this means in practice.

Each "Geth" program is identical. When given the same inputs, they reach the same conclusion, and thus, act in concert. But "when given the same inputs" is actually a pretty hard thing to achieve! When Geth "achieve consensus", they are sharing the data each individual Geth platform has recorded so that the programs within come to the same solution.

But the networking Individual Geth platforms do not usually have the processing power to achieve consensus in this way -- it's why the Heretic Geth installation (from Legion's loyalty mission in ME2) starts with no enemies -- the Geth are 'living' in servers, where they have the numbers and hardware to perform this processing more efficiently. Source

An important take away from this is that the Geth hivemind is neither instant nor omnipresent. Until they have a chance to link back up to a large enough network, two Geth in adjacent rooms do not know what the other sees.

What makes Legion unique is that it has enough Geth programs (10x the normal amount) that it is able to "achieve consensus" locally. So when Commander Shepherd shows up, all of the Geth platforms go: "Hey look! Commander Shepherd!" But Legion alone can go one step further and decide, "Oh, we shouldn't shoot them!"

It's plausible (assuming Legion uploaded its experiences from ME2 to the Collective) the rest of the Geth would come to the same conclusion -- but they're not able to "think" on the spot. And because Shepherd has a tendency to destroy any hostile platforms they come in contact with, it's unclear if the collective at large would even know Shepherd had arrived on the scene!