幽霊

Do I disappoint you? Am I not what I seemed? I get disappointed too, when love is not what I dreamed

Aside from Riven's entire speech about Mara not being upfront with us, I've never trusted her all that much. In Season of the Lost, she severed Savathun's connection with her worm, which allowed her to die outside her thrownworld and receive the Light. In a lot of ways, she was responsible for the Witch Queen campaign. And, to be fair, we were all tricked by Savathun in that season, but we had a reason to bargain with her. She had Osiris held hostage. But why was Mara so eager to help Savathun? Last I checked, she's got no reason to risk it all for Osiris (I might be wrong, but I'm not sure if they even knew each other before this). Maybe she did it to get her techeuns back, but why make a deal with Savathun to do it when the Guardian would likely be willing to help her out with that anyway? It just seems like a lot of risk for an unclear motive. The more we look at it, the more we find Mara assisting us while having very unclear motives. A lot of her actions just don't add up. And while I have absolutely no reason to trust a single thing Riven says, she does make a good point. We don't know what Mara's getting out of this. This is the second time she's made a deal with one of our greatest enemies without being clear about what she stands to gain. We know her overarching motive is to remove the curse on the Dreaming City, so maybe she's trying to accumulate power to do so? And if that's true, how far is she willing to go?

Mara also had a dual incentive to help Osiris- to start, she and Osiris have actually worked together in the past (though mentions have been fleeting), so Osiris is one of her allies, perhaps even a person she considers a friend. Second, it allowed her to get back at Savathun in the process, since Savathun is the cause behind the curse on the Dreaming City (it was her wish with Riven, iirc). Savathun didn't survive the process and it was unknown to all of us at the time that she was eligible to be a Lightbearer, so Mara DID succeed in killing Savathun- she just didn't realize she'd come back from it.

I'd also add that Mara's goal has always been and remains to be saving humanity. This is the sole reason she made the Awoken as she did back in the Distributary, to make them willing to leave the safety of the Distributary and return to the system to help humanity. She considered the Distributary to be a temporary haven to escape the Collapse and that the Awoken owed a debt to the rest of humanity for surviving the Collapse inside the Distributary; a debt they would have to repay and one day return to the system:

"It is time that we accept our debt. The Distributary is a refuge, not a birthright; a base to rebuild our strength, not a garden to tend. I ask you, Awoken, to join me in the hardest and most worthy task a people has ever faced. We must leave our heaven, return to the world of our ancestors, and take up the works they abandoned. If some of them survive, we must offer aid. If they have enemies, we must share our strength. We must go back to the war we fled and face our enemies there."

I'm not sure why people forget that; possibly because it was framed in Marasenna as something that made Mara "the devil" because she schemed for this to be possible, as well as for deliberately making "imperfect" people. What people forget is that she had to. If she made the Distributary and the Awoken differently, they never would've left. Why would they?

"I have nurtured and tended the Eccaleist belief so that there will always be Awoken who feel uncomfortable in paradise. Guilty for the gift of existence in the Distributary. People who'll come with me."

She is correct, of course, albeit she did this in the most convoluted and shady way possible. But she was correct in doing so, because the Awoken were needed by the rest of humanity, and still are. Mara's actions further saved humanity at Twilight Gap and with Oryx's arrival. Both of these events would've been devastating to humanity, and possibly would've wiped us out (Oryx sure would've).

Mara has always been our ally, despite her schemes and weird ways of achieving her goals that often hurt a lot of people in the process. She used to be the sort of "end justifies the means" type, but has since changed as she started working more closely with us and, as the above reply mentions, since she had a personal run-in with the Witness. Ikora's assessment of Mara from the Witch Queen Collector's Edition:

I believe Mara has begun to consider that she may not be the prime executor of her own endgame. She may be just one component of the bomb—a payload or a timing device. At the end of her own journey, she is necessary but not sufficient. She can no longer fight alone.

This is why she helps us. She has grand plans to save humanity, but she realised that she cannot execute those plans alone. She cannot fight alone and she cannot win alone.

And of course, to reiterate, in regards to Savathun; nobody knew or even considered the possibility that killing Savathun would actually help her. This was a massive and incredibly wild event that happened in-universe. Mara killing Savathun was Mara using her incredible power to strike down one of our most insidious enemies and it worked. Nobody, not even Mara, could've known that this is what Savathun wanted and that she would be granted the Light. Like, I need to stress that the Hive becoming Lightbearers has to treated as a world-shattering event that it was. Nobody knew this was possible. It was on nobody's radar as possible. Imagine the least likely situation that could never happen on Earth and that was Savathun being rezed by a Ghost.

In the first chronological appearance of Mara, while she's still human and on board the Exodus ship, one of the first things she talks about is how much she loves Earth:

She was nineteen years and nine months old at the moment the ship began its transtellar injection burn, although this is true only if you count by the calendar of a planet she has barely visited but will always love. She thinks you cannot help but love Earth if you grow up in space. You love Earth the way all adolescents secretly adore two-century-old video of nai nai and ye ye dancing on New Year's Eve. Earth does not ask too much. The colonies are demanding parents, but Earth is like a chill old grandma, simmering in weird art and weirder ideas, enthroned upon ecology older than Human time. Earth was the first terraformed world. Life made Earth livable.