I have to say this so many times, NOT EVERYONE WHO KILLS. IS A YANDERE.

Let me explain what a Yandere is;

A person who is sweet and cute on the outside, with obsessive and/or murderous intent on the inside

Let me repeat that,

A person who is SWEET AND CUTE ON THE OUTSIDE, with obsessive and/or murderous intent on the inside

Everyone seems to forget the "dere" part of a Yandere.

This means characters such as;

-Homura Akemi
-Ayano Aishi
-Legosi

Are NOT Yanderes! You may say "But Ayano Aishi' sa yandere!" No she's not. She's a "yan" because she has no "dere" to follow

Also, not all Yanderes are murderers. Some of them are just obsessive.

I've seen the "Homura is a yandere!" So much because she's obsessed with Madoka's promise and wants to protect her, even if it means killing people that will make her upset and make her turn into a magical girl. Homura's a kuudere, you fuckhead!

I've seen people call Legosi a yandere because he's willing to get blood on his hands in order to keep Haru safe. He is literally nothing like a yandere.

Besides, I personally believe Yanderes don't need to have a lover. It could be anyone! I made a character in Royale High that was sweet and friendly, but murdered all of her friends and trapped their souls in her locket so they could be with her forever.

She's a yandere.

I've seen people calling Dust Sans a yandere, HE'S JUST A KILLER, THERE'S NOTHING DERE ABOUT HIM!

So when writing a yandere, keep in mind the "dere" part of them :)