You might be confusing Nephandi (the Fallen) and Marauders (the Mad).
A Nephandus was either born with a corrupted avatar or willingly chose, along with their avatar, to be destroyed, remade, and inverted, to permanently discard any hope of Ascension and embrace Descent. They are not insane. Gilgul destroys the avatar in the hope that it won't be reincarnated, and execution is usually next. Nephandi are basically the ultimate “bad guys” of Mage.
A Marauder is a particular kind of insane Mage that exists in a permanent state of Quiet whose madness affects reality around them. Despite what you could do to help them, their Quiet isn't going to go away and isn't something you could directly affect. I suppose it's possible to gilgul one and be left with an insane Sleeper, but more often they're causing enough trouble that players are trying to merely contain them until their Quiet whisks them out of our reality. Marauders are basically plot devices in Mage.
Any sort of recovery for a Marauder is a big question mark that a Storyteller could design an entire story around. That could include gilgul and then some sort of restoration of their avatar. M20 hints that undoing a gilgul might be possible.