For those of you who don't know, fandoms are communities of people who appreciate and enjoy a work of some kind, normally a television show or book series. Fans make art, write fan fiction and talk to other people who enjoy the same work. This is a great way for people to make like-minded new friends. However, fandoms can get very toxic, very quickly.
The most prominent example of fandoms becoming toxic is from the "Steven Universe" fandom. A young girl, who was a fan of the show, enjoyed creating art for it and sharing it to Tumblr. Other fans of the show decided that the way she drew the characters was so "problematic" that they bullied and harassed her to the point of attempting suicide. The problems that people had with the characters? They were "too thin," or "whitewashed."
While it is OK to want the characters to look the way they do on the show, artistic expression and interpretation allows people to draw the characters however they want. If you don't like it, you don't have to look at it, and you certainly don't have to bully somebody over it.
Other parts of fandoms have similar problems. "Shipping" characters, or wanting two characters to be in a romantic relationship, causes a lot of people to become upset as well. Again, it's understandable to not like the idea of two characters getting together, but it's a better use of time to either ignore it, civilly discuss why it could be uncomfortable for people to hear that or simply not look at it.
The "Steven Universe" fandom isn't the only fandom that becomes toxic very quickly. The show "Supernatural" had a fandom that would generally not accept any criticism of the show and threaten people if they did. The series "Attack on Titan" had fans telling other fans to kill themselves over ships. Even the "Harry Potter" fandom gets heated over shipping and headcanon. Many fan artists, or writers, get messages full of hate and even threats to their well-being for something as simple as drawing a character with a different hairstyle or writing two fictional characters into a fictional relationship.
Enjoying a show and talking about it with others can be a great experience, and being passionate about ideas is wonderful. The passion cannot get to the point where other ideas cannot be accepted or tolerated. Nothing is worth telling a person to kill themselves over; not religion, not politics and certainly not fan art or fictional characters.