Some of them might laugh. They would see the creative and sometimes out - there ideas that fans come up with in the fanfiction. For example, if there's a fanfic where Oliver Queen teams up with a really strange character from the Arrowverse, it could be quite humorous to them.
The Justice League as a whole would have a diverse range of reactions. Some members like the Flash would be all for it, seeing it as a fun exploration of Batman. Others like Wonder Woman might use it as a way to understand how the public perceives Batman's role in the League. And then there are those like Aquaman who might just shrug it off unless it directly affects his interests. The fanfiction could also potentially create some internal discussions within the League about how they are all portrayed in the public's imagination.
The Flash might initially be in denial. He's used to Batman always having a plan and his death would be a huge shock. He would then turn his speed into a tool for investigation, trying to find out exactly what happened. Green Lantern could feel a sense of loss but also be spurred into action, using his ring to try and prevent future tragedies like Batman's death. He might create constructs in Batman's memory to inspire the team.
Some of them might be amused. They could find the different takes on their characters and stories quite entertaining. For example, Superman might chuckle at a fanfic that has him in a comical situation.
They would be amazed. The Justice League is used to their own brand of super - powered battles, but Infinity War would show them a whole new level of cosmic warfare. Superman might be the first to jump into action, mentally preparing himself to take on Thanos if he was in that universe.
They'd likely be intrigued. Batman would study it for plot devices and character development. He might use some of the ideas in his own crime - fighting strategies. Superman would probably be entertained, seeing it as a new form of storytelling about a hero. Wonder Woman might view it as a cultural phenomenon and think about how it reflects on modern society's view of heroes.
Batman would probably be more critical. He'd analyze how the fanfiction portrays his detective skills and his relationship with the other members of the Justice League. He might not like it if the stories make him seem too soft or too brutal, as he has a very specific self - image.
Some might be amused, while others could be offended. It depends on how they are depicted in the fanfiction.
He'd probably be confused at first. Naruto is used to being among friends and comrades. But then, his determination would kick in. He'd likely try to talk to them, to understand why they think he's a threat.
Some might be shocked. For instance, Batman is often portrayed in very different ways in fanfiction compared to his 'canon' self. Superman might raise an eyebrow at a fanfic where Batman is overly emotional. Others like the Flash might be really into it, finding the different takes on Batman's character and his adventures exciting.
They might be quite amused. The Justice League members, with their own complex backstories and powers, would see different interpretations of the Arrow universe in the fanfiction. Some might find it interesting to see how fans envision the crossovers or alternate storylines that could involve Arrow characters.