Why is Meiko always the bad guy???

This is like asking....
Why does Len always die?
Why is Rin always bossy?
Why is Miku always sweet?

Going off of what zerowakakkoi said, and especially since you're talking about fanfictions and not songs; basically the popular headcanon of Meiko is that she's older than most of the other Vocaloids, drinks, and isn't totally a nice person.

Also, Meiko, while a lot of people know her; she's not as popular when compared to the other well known Vocaloids (Miku, Rin, Len, etc.) so people are probably also more willing to make her the villain than other Vocaloids that they like, that's kinda my own personal speculation.

Long story short, don't take peoples hate comments against Meiko personal, it's just like you say; it isn't true. Crypton has not given any of their Vocaloids ANY canon personalities, traits, items, etc. there are just things, like Meiko's sake, that are widely accepted headcannons.

I dunno. Meiko is cool

I know right? No matter where I go it's always Meiko being evil. I don't think she's actually evil. I think she's actually a nice person.

I don't really know since she is very motherly and kind , but probably just some idiot haters or Mikutard . I guess she is being the bad guy in KaiMi fan fiction right ? . but she wasn't always being a bad guy , for example read some KaiMei fanfic.

They're n00bs. They're going to put a lot of clichés in there, especially Meiko being the bad guy and Miku being the good girl. If you headcanon the opposite, then that's okay. Publish it on the web if you want to.

IDK. Maybe she's not popular or that the Vocaloid staff don't like her.