Lucy would be completely devastated. She and Natsu have a very close bond. She might go into a deep depression, constantly blaming herself for not being able to save him. Gray would be angry at first, feeling like he should have been there to help Natsu. Then he'd be sad and would probably vow to avenge Natsu. Erza would be strong on the outside but inside she'd be broken. She might try to keep Fairy Tail together in Natsu's memory. Happy would be inconsolable, constantly flying around looking for Natsu and refusing to believe he's really gone.
The other members of Fairy Tail would be in shock. Wendy would try to use her healing magic, even though she knows it's too late. She'd cry and feel like she let Natsu down. Gajeel would respect Natsu's sacrifice if it was for a noble cause but still be sad. He might say some gruff words about how Natsu was a worthy opponent. Makarov would be heartbroken as he saw Natsu as one of the guild's most important members. He'd probably hold a big memorial service for Natsu. And the rest of the guild would be in a state of mourning, with the whole atmosphere of Fairy Tail becoming very somber.
When Natsu gets angry in the fanfiction, different characters have different reactions. Erza, being the strong and authoritative figure in the guild, would likely try to discipline Natsu in some way, but also understand that his anger often comes from a place of protecting his friends. Wendy would be a bit more timid in her approach. She might use her healing magic to try and calm Natsu's emotions, as she can sense the turmoil in his magic energy. And Makarov, the guild master, would probably give Natsu a stern look and a lecture about controlling his temper.
Gray would be shocked at first. He and Natsu have a long - standing rivalry - but also a deep friendship. He might initially be angry that Natsu would just leave like that, but then he would realize how much he'll miss him. He might also feel a sense of loss for the guild as a whole, since Natsu is such an important part of it.
Lucy would be extremely distraught. She and Natsu have a very close bond. She might cry for days and even isolate herself from the rest of the guild at first. Gray would be angry and feel a strong sense of loss. He'd probably blame himself for not being there to help. Erza would be stoic on the outside but inside, she'd be deeply affected. She'd take charge of the situation, making sure Natsu's death was not in vain and that the guild stays strong.
Some members might be sad at first. Lucy, for example, would surely miss him a great deal as they are close friends. She might worry about him constantly during his absence.
The guild members' reactions would vary greatly. Happy, being Natsu's closest companion, would likely be excited about the fanfiction. He might fly around the guild hall, telling everyone about it and adding his own little comments about how accurate or inaccurate it is. He would definitely be protective of Natsu's image in the fanfiction, especially if it's not a positive portrayal.
Gray would be angry and sad at the same time. He was Natsu's rival but also his friend. He might lash out at first, blaming the situation or other people. But then he would turn his anger into a determination to find out what really happened to Natsu and seek justice if there was foul play involved. And he would also be there for Lucy and the other guild members, trying to keep them strong.
He might initially be shocked and confused. Natsu has always been so loyal to Fairy Tail, so this would be a huge blow to him.
Most of the Fairy Tail members would not take it well. Happy, Natsu's closest friend, would be extremely confused and sad. He'd follow Natsu around, trying to get him to come back to the guild. Makarov, the guild master, would see it as a huge threat to the guild's safety. He might send some of the stronger members to keep an eye on Natsu and Zeref. And the younger members like Wendy would look up to Natsu so much that they'd be very disappointed and scared for him, thinking that he's making a big mistake by joining Zeref.
Natsu would likely be extremely hurt. He has always been loyal to Fairy Tail, so being betrayed would be a huge blow. He might initially be in disbelief, not wanting to accept that his friends and guildmates could turn on him.
The guild would likely be shocked at first. They consider Natsu as one of their own, so his attack would be completely unexpected. Some members might try to talk him down, like Lucy. She has a close bond with Natsu and would probably try to reason with him.