Perhaps the Marauders wanted to warn Harry about something important and thought a howler was the best way to get his attention. It could also be a part of a complex magical game or challenge among them that accidentally involved Harry.
Well, it could be that she broke some important rule or did something that really angered another character, and they decided to send a howler to give her a piece of their mind. Or perhaps it was a misunderstanding that led to the howler being sent to her.
Well, it could be that someone was really angry with him and sent it to express that rage.
He might be initially embarrassed, especially if he receives it in a public place like the Great Hall.
Well, Sirius could get really flustered. It could lead to some comical or tense moments depending on the context of the fanfiction. The howler might reveal something he doesn't want others to know.
Harry could be angry at first. He might think his parents are overreacting or don't understand his situation at Hogwarts. But then, after thinking about it, he realizes they are just worried about him and starts to feel guilty.
Voldemort might initially be furious. He's not used to being challenged in such a blatant way. He could try to destroy the Howler immediately in a fit of rage.
Maybe it happened because he made a big mistake in a magical experiment or broke a strict school rule. That might have triggered a howler being sent to him. The possibilities are endless in fanfiction!
Harry might be initially shocked and embarrassed. A Howler is a very public form of scolding, and in front of all his friends and classmates, it would be a humiliating experience for him.
It could be that someone did something really wrong and Harry needed to express his strong feelings in a very public and intense way. Howlers are notorious for being attention-grabbing and forceful.
Snape sending Harry a howler in fanfiction can vary greatly. Sometimes it's a turning point for their feud to ease, while in others, it escalates the tension even more. It all depends on the author's imagination and the story's direction.