Why is being a Silent Brother considered evil?

Question

In the world of Mortal Instruments, when Raziel (and further plot events) explains how Glorious works, it appears that the holy fire burns away everything evil in the person, along with the person itself if the person is more evil than good.

However later in the story, we see that Brother Zachariah, when exposed to the fire, losses whatever was making him a Silent Brother and remains a pure Shadowhunter. This in essence implies that being a silent brother is evil, as it gets removed by evil-removing effect. However as Silent Brothers are aides of Shadowhunters since always, why would that be true?

Answer 1

The silent brothers are clearly augmented above the level of an ordinary shadowhunter. To do so would require some sort of more secretive or dangerous runes. These runes could use the powers of evil to augment the silent brothers, while the shadowhunter within remains pure. It's shown that silent brothers (Zachariah specifically) lose a good deal of their humanity, a problem which is solved when Glorious burns the runes away. Therefore, it is most likely that Glorious is destroying the more sinister things that helped to form the silent brother rather than cleansing the being within.

Answer 2

Read The Infernal Devices.

Brother Zachariah only became a Silent Brother because of a Demon-inflicted addiction to Yin Fen. If he hadn't become a Silent Brother he would have died,which would have destroyed Will Herondale and Tessa Grey. In City of Heavenly Fire, Brother Zachariah tries to heal Jace. However, when he touches him, the Heavenly Fire is absorbed. This burns away the Yin Fen, thus meaning that the runes that needed to save him are reversed, turning him back to a regular Shadowhunter.

It is also stated that Zachariah is more human than most of the Silent Brothers, as he wasn't able to take all of the runes. That is why he looks different. He is also just as powerful as the other brothers, as he and Tessa resided over the protection ritual for Clary.

Silent Brothers are also more angelic than regular Shadowhunters. This is because they have sacrificed some of their humanity for more Angelic power. This allows them to bear marks that would normally be locked for other Shadowhunters. There is nothing evil about the Silent Brothers, they have just chosen a different path to the rest of the Nephilim.

"In exchange for their special abilities, they have given up some of their humanity, moving farther from the Earth and closer to Heaven than the rest of us." - The Shadowhunters Codex

Normal Shadowhunters can only bear the angel's runes that are found in the Grey Book. However, the only 'evil' runes that we ever see, are those worn by the Endarkened, and the Rune that binds Sebastian and Jace.