We all know Sage, the virtuous, honorable, flawless hero of the Valorant world. She has no ill thoughts towards anybody and holds no grudges, she only thinks about saving the people and being a good hero. However, there is surely a dark story about her. Alas, nobody is perfect and nobody would have such courage to act like a flawless hero. I think she is insecure and afraid. Thus, to hide her weak personality and dark secrets, she adopted a "brave" persona. We can understand this by listening to other characters.

First, lets talk about Omen and his voice lines towards Sage:

"Sage, the life you give...do you ever wonder where it's taken from? "

From this voice line, we can understand that sage heals people by sacrificing or doing something that she does/does not know about. If we take this Lore Theory about Omen by u/thebirdof_hermes as canon(which probably is). Omen may be the source of Sage's abilities or he knows something about Sage that we don't know about.

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"A radiant healer is with them. I can feel her pull against the natural balance."

This voice line from Omen also tells that Sage definitely is causing some damage to the balance of nature by healing people.

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We also know that Viper was a healer before, and Viper, Omen, Sage trio worked together back then, which can be understood from the voice lines between the trio:

Viper to Sage: "That's payback Sage."
Omen to Viper: "Sabine, look at the monsters we've become! At least you have flesh to hide your horror."
Omen to Viper: "Just think, Sabine! You used to heal with your skill. Funny."

Sage has no direct voice lines towards Viper and Omen, which means that she knows she is guilty and she does not have the face to speak to them.

We can also assume from the theory above that this trio caused an unintentional catastrophe before.

Viper to Sage: "Sage, you're the only one who can keep us alive. Don't fail us now like you failed me then."

From this voice line, we can tell that Sage was the most important factor in this catastrophe and her failure lead to Viper being a mad scientist who meddles with chemicals and caused Omen to lose his body and become the void.

She failed big time, she is insecure and afraid but she hides her fears behind a heroic persona. She is keeping that persona because nobody knows (except Viper and Omen), and she doesn't want to reveal her secrets, because everybody could hate her. The hero of the people, the symbolism of honor.. What would people think, if it was all a lie?