This label and title appears to have been a dominant facet of Forerunner and presumably Precursor culture, and likely played an important role in the Forerunner's war against the Flood. While it's context is self-explanatory, there are aspects of it that may have been overlooked.
We know that it was looked down upon by the Forerunner in their later days, and seemed to have been partly responsible for their downfall. The Librarian goes as far to say that it "bred dependence and sloth" and rendered the galaxy helpless in the face of the Flood, as if she was renouncing it.
But on the other hand, she and many other Forerunners go about indexing and saving other sentient life and bringing them to the Ark, obeying the ideology that seems to underly the Mantle. In other words, she contradicts herself; she decries the Mantle as being a negative property, but she heads an effort that fulfills and is in accordance with that same flawed ideology. Why follow an ideology that has produced nothing but death, disappointment, and catastrophe?
Perhaps it is because that ideology is deeply ingrained within the Forerunner's psyches. If people live with a strong ideology pervading their everyday lives, and even if they seem to reject or doubt it, it will still have a strong impact on their behavior and activities. Given the prominence of the Mantle, it likely had an important role in Forerunner thinking, especially in the war with the Flood.
That said, was the title one truly taken acted out for the sake of nobility and compassion, or does hide a darker, more negative seed?
Given their rise to power and dominence within the galaxy and their 'inheritance' from the Precursors, it stands to reason that their views of themselves was likely extremely high, perhaps to the point of narcissism. Since no other race at the time could equal them, this would only further serve to boost their egos and serve as evidence of their superiority.
If one says that he/she/it must protect something, that implies some sort of superiority or advantage over the defended agent. The defensive agent is protecting something that cannot defend itself, as it is entrusting its safety to the defender.
In that light, the Forerunner probably saw themselves as superior to all other life in the galaxy and appointed themselves as galactic guardians, likely seeing other creatures as in some regard inferior to them. In this regard, the Forerunner would self-absorbed and even narcisstic.
Why? If every race in the galaxy needs their protection, that implies that these races cannot fend for themselves, cannot think for themselves, and are effectively blind, defenseless infants that need to be coddled.
Too add insult to injury, they pass the Mantle onto the Humans of Earth, who if theories are to be believed, are dislocated, primitive cousins of the Forerunner themselves. Going further, the Forerunner give them this title because of their evident relation and similarities, and hold them above other species.
But isn't this the same as saying that this type of life form is superior to all others, that only those who are 'human' deserve to hold great power, and that all others must bow before them and worship them? How is that fair?