The main cause could be a prophecy. Natsu has learned about a prophecy that foretells some great disaster, and he feels powerless to stop it. As he sees the signs of the prophecy coming true, the stress and fear build up until he snaps. He then decides to take extreme measures to try and change the course of events.
Maybe it's a fellow guild member who has made a series of bad decisions. Their actions keep putting the guild in danger, and Natsu has been warning them. But when the situation gets really bad and people get hurt, Natsu snaps at that guild member for their irresponsible behavior.
Perhaps in this 'natsu finally snapped fanfiction', Natsu snaps due to some dark magic influence. He has always been a fiery and passionate character, but this time the dark magic takes over his emotions. He starts to turn on his guild mates, believing they are the enemy. It could lead to a big fight within the guild, with everyone trying to figure out how to save Natsu from this dark state.
It could be an evil wizard. There are many powerful and malevolent wizards in the Fairy Tail universe who might target Natsu for some reason and use dark magic to make him cold.
Lucy could be a main character. Since she's often by Natsu's side, she would likely be deeply involved in his recovery. She could use her celestial spirit magic to try and protect him while he's injured or search for a way to heal him.
It could be Orochimaru. He has always been a source of trouble and if he manages to pull off some really devious plan that directly affects Naruto or his loved ones, it might cause Naruto to snap.
Definitely Ruby Rose. Since she's the main protagonist in RWBY, it's likely that Natsu would cross paths with her first. Their personalities are quite different, with Ruby being more innocent and Natsu being more brash, so their interactions would be interesting.
He could marry an original character created specifically for the fanfiction. This character could be a powerful mage from a far - off place who crosses paths with Natsu during one of his adventures.
It could be a jealous rival from another guild. They might see Natsu as a threat and figure framing him would be a good way to get him out of the way.
I'm not sure who exactly Natsu is just from 'natsu reads fanfiction'. It could be a normal person who just enjoys reading fan - made stories. Maybe it's a nickname for someone within a particular fan community.
There could be a female dragon named Ignis who is very fiery and passionate, always trying to be the closest to Natsu. Another could be a more gentle dragon named Aqua, who has water - based powers and uses her calm nature to balance out the more intense dragons in the harem.
One of the main characters could be Happy, Natsu's loyal flying cat companion. Happy would likely be by Natsu's side during his retirement, still providing his comical relief and support. Another character could be Lucy. She might be concerned about Natsu's retirement decision and try to understand his reasons, or she could also be retiring from her own magical adventures and join Natsu in his new peaceful life.