FanFiction is an odd topic. There's a lot of lines where I'm not sure if there should or shouldn't be any. I'm still mixed on the smaller details about it.
On one hand, it's a great way for those who've just picked up writing to explore said writing without being caught up in the details of creating an entire world, all of the characters, etc. It's a template. You also get an audience that's already interested because it existed before you started. Fans get to see more content of the universe they love, and writers get more free advertisement.
Overall, Fanfiction is harmless. It can be fun, there's lots of creativity, and for some writers they simply create better in Fanfiction than on their own and that's why they dabble in Fanfiction. It's almost like co-writing, but without the direct partnership. If it works, it works!
On the other hand, I always found it awkward as a reader of Fanfiction when characters were OOC. The writers I came across didn't seem to give any thought to trying to re-create them, just re-write them. On some level, it feels disrespectful to the originator (the canon writer). Like taking Character A, who's very polite and compassionate, and then turning Character A into a violent bad boy because....reasons.
But, again, I think that's where my own thoughts on it gets confusing and contradictory for me. The best thing about writing is the creative freedom. I think that should apply to Fanfiction as well? At the same time, a template is a template for a reason. And if the characters are not remotely in character (even that can be subjective! The writers could feel they're in character for the specific AU, while I don't, so my opinion on that matter is moot), what is the point of using the template--the characters, the world--at all? I understand writing it anyway, but I guess I don't understand reading it as a reader. 
Where I don't think Fanfiction writers should be targeted in any way, as others mentioned, Fanfiction of real people is extremely weird and shouldn't happen at all. For a variety of reasons. It's one thing to make a fan portrait of real figures somebody admires. It's another thing entirely to make up stories about them.