Well, in Naruto fanfics, 'it's for a good cause i swear' might refer to a situation where a character breaks the rules. For example, Sasuke might join forces with an enemy group, but the author wants to show that it's for a good cause such as getting stronger to protect the village in the long run. It's a way for the writer to justify certain actions that deviate from the norm in the Naruto universe.
A Naruto fanfic can be for a good cause if it promotes positive values like friendship, just like how Naruto values his friends. For example, a fanfic could show characters from different villages coming together to help each other in a time of crisis.
It could be an old enemy who has come back with new and more terrifying powers. For instance, Orochimaru might return with some kind of jutsu that can directly target Naruto's mind and create unfathomable fears in him.
In some fanfictions, the cause could be a curse. There could be an old enemy of the village or someone with a vendetta against Naruto who placed a curse on him. This curse could be slowly draining his life force, causing him to become sick. Or, it could be related to the tailed beast inside him. Maybe there was some imbalance in the chakra of the Nine - Tails that was affecting Naruto's health. It's also possible that he caught a strange and rare disease from an unknown source during one of his travels to a far - off land.
It could be a new villain. Maybe a villain who targets Naruto's loved ones and Naruto is unable to save them. This failure would cause him great heartbreak. For instance, if the villain kidnapped Hinata and Naruto couldn't rescue her in time.
Yes, in some fanfics Naruto might be depicted as swearing a lot. Fanfic writers sometimes take creative liberties to show different aspects of a character, including adding edgier language. However, this isn't in line with the original Naruto character in the official series, where he is more positive and less likely to use a lot of swear words.
One reason could be false accusations. If Naruto is wrongly accused of a major crime in the village and no one believes him, he might start to hate Konoha.
Maybe he has been through some extremely traumatic event. For instance, if he lost all his friends in a single battle, it could be so overwhelming that he shuts down his emotions to cope.
Perhaps it represents four important characters in the fanfic. Each '1' could be associated with a different character's journey or ability. For instance, one '1' for Naruto's growth, another for Sasuke's redemption, and so on.
It could mean that in the fanfic, Naruto has some sort of plan or scheme to bring about destruction. Maybe he's been pushed to the edge or influenced in a way that makes him want to cause chaos. It might be a very different take on his character compared to the original Naruto series.
If Jiraiya made a deal with the village elders that put Naruto in a difficult position without his knowledge. For instance, agreeing to send Naruto on a dangerous mission without his consent just to gain some political advantage for the village. Naruto would be angry at being used and not being informed by Jiraiya.