erwin gets his arm bit off

No, he doesn't. Tyrese keeps his arm intact throughout the comics.

Maybe it's a survival story where the man had to do it to escape a dangerous situation.

In Pulp Fiction, the one who gets his head blown off is Marvin. It's a sudden and shocking moment that adds to the film's gritty and unpredictable nature. The event occurs due to an accidental gun discharge.

Generally speaking, it was a common plot in novels that an arm would not spurt blood after being cut off. In fact, after an arm was cut, it would cause intense pain and bleeding, but it would not spurt blood like in the novels. If the arm is seriously injured, a lot of blood may flow out, but this is usually controlled by blood clot and hemostatic measures after the wound ruptures. Therefore, if one wanted to write a story about a person spurting blood after his arm was cut, he had to carefully consider the plot and details to ensure that it was true.

Maybe Orochimaru. Despite being a villain in the past, he is a master of forbidden jutsus and experiments. He could see Naruto as an interesting subject and give him a new arm with some dark powers, which Naruto would then have to learn to control for the greater good.

I'm not entirely sure which 'arm fall off boy' you specifically refer to. It could be a very specific or niche character from a certain comic, game, or local story. Without more context, it's difficult to detail the origin story.

It could be a high - ranking and strict angel who believes aziraphale has strayed too far from the angelic path. This angel might see aziraphale's interactions with Crowley or his unorthodox ideas as a threat to the order of the celestial realm.

In the novel The Return of the Condor Heroes, Yang Guo's right arm was cut off instead of his left arm. This plot happened when he was fighting with the Little Condor. Because the Little Condor attacked him, he slashed his right arm to protect himself and his loved ones, causing his right arm to be disabled.

Well, perhaps the new arm comes from some kind of ancient and mysterious source. It could be an arm that was once part of a powerful ninja from the past. This new arm could have a mind of its own, at first causing Naruto some trouble as he tries to control it, but eventually they work in harmony and it gives Naruto an edge in battles against tough enemies.

Maybe he was protecting someone in Wave. He's always the self - sacrificing type, so he could have put himself in harm's way to save a villager or a friend.