Legion died because he lost all of his previous principles.. he used to think that the Geth had to build their own future. Once he was given Reaper code, he was brainwashed into thinking only the Reapers could give them a future.
As far as I'm concerned, he never gives the Reaper code to other Geth. I just shoot him myself.
Eh, he's just our (Gospel of, the gnostic version) Judas.
I do agree with this now being the same Legion. But really, as far as synthetic minds go, he only just 'changed his mind', after coming into contact with more information.
The earlier example is Mordin choosing to outright cure the genophage instead of doing a controlled modification of it again.
"I made a mistake!" (Mordin finally moving away from justifying the Genophage, and justifying only fixing it to an extent)
In both cases, you're encouraged to allow something huge to happen that will fix so many problems.
But at least in Cure/Upload, unlike Synthesis, we can see some negative or at least lacking outcomes from not doing everything 'correctly'. I'm not sure there's much that seems lacking from any version of Synthesis. Eh I guess the Krogan/Quarian/Geth stuff counts.
This is all part of the 'green' layer of the story though, imo. Most versions of Genophage outcome provide the at least minimal foundations for picking Synthesis, but then also the Upload/Cure, Thane and maybe his romance, and some other stuff.
Of course, all of this stuff is the most highly faith based. Faith that the cure won't doom the galaxy to the Krogan again. Faith that the Geth won't go wrong after being upgraded/being integrated in Quarian suits. Faith that you'll meet Thane across the sea. Faith that synthesis is possible, and we can all live in peace.
I think its just a dream. But a nice one, mostly. In ways, better than the RenegadeStuff+RuinedDestroy nightmare 
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Legion 'died' because he had to quickly (a lot of this stuff is time-constrained in the plot, like Mordin needing to go up that elevator NOW, for example) send literally himself to everyone.
If he sent a copy of himself, it may have gone wrong. Or rather, creating a copy of the now gestalt-code-entity he was in ME3, resulted in an error. So he had to do a full broadcast - a dissemination.
What Reaper code does to a geth platform is unify all of the programs into a single larger program that can more easily be determined (by some organics at least, and maybe some synthetics) as 'life', or at least a new form of it.
So what Legion is in ME3, isn't something that can be copied easily. It is life itself.
It would be as if everything of Shepard had to be copied, without the body/brain/etc involved. It would be impossible to have it fully transfer over. Hardward and software. This is why Control has ReaperShep as so different - it is same software but different hardware.
So direct personality dissemination is not suicide. Not in the transcendent sense at least. In that sense, it is actually eternal life, living on in another form in every geth platform/hub. A shared consciousness (or even sub-consciousness) that unifies the geth, and has them carry the (not just 'software', but 'hardware') memories of GethVI/Legion.
In the more objective sense, it is suicide, and one that Shepard may not approve of or allow at all.
In the more subjective sense, it is only kinda sorta suicide. It is understandable what he's doing, but not necessarily something to approve.
But in the transcendent sense, a Legion during Peace scenario doing this is not suicide. It really is like taking pieces of himself and putting a bit of it in every 'geth's' (each collection of programs now a 'person') mind. And it'll be what allows the geth to communicate much more easily and openly with Quarians and other organics, without always being on bad social footing. The Consensus system (and pursual of Dyson Sphere) was great in getting the geth to a certain point in intelligence, but eventually proved to be obsolete, and we had to choose whether to embrace the change, or to let the Quarians put the Geth down.
It is space magicky. Even the Genophage cure is somewhat more space magicky than normal (before ME3). That's just how this story path rolls I guess.