So, i'm currently outlining my new story: Training Diary of Sayori-sama.
And, the MC's fiancee, Sayori is a yandere. What i want to do is, she secretly hatches a plan to mold MC into the perfect groom under the guise of "vacation" to their family's summer house. I need some tips on how build up an extreme yandere without killing the MC in the process.
That's not what yandere do as a rule. Yandere are basically just people whose extreme love warped their decision-making abilities. They love the other person so much that they want to be with the person so much and removes all barrier or interruption between them and their other person.
That means there is a very, very narrow circumstance that leads to them killing their beloved. Usually it's something along the line of "I'll kill you, then I'll kill myself, that's the only way we can be together," or "I'll kill you and keep you by my side forever, so that you will never leave me again." But the "Hurr durr, don't kill me, please" is mostly in reference only to Mirai Nikki.
In general though, yandere do not kill their love target. They may maim, cut off the limbs, kidnap, rape or imprison their love target, but never kill them. Because you see, if you kill your love one, then they're dead. What are you going to do with a dead body? They would however, kill everyone else, because if everyone else is dead, then there're only them and their target and they can live 'happily ever after'.
In writing a yandere, you need to:
1) Put yourself into the shoes of an unhinged, psychotic character who feels extremely insecure any time your love target looks at or speaks to another person.
2) You need to look into yourself and imagine what would happen if your target, for this case, we shall assume a man, sleeps with another woman, puts a baby in her, and leaves you while laughing. In reality, this doesn't happen, but this is what you imagined whenever you see him speaking with another woman.
3) Imagine what you would happen if the love of your life leaves you while the woman in his arms laughs and ridicules you, for not being good enough or saying that he never loved you and he only loved her all this time. This is not true by the way, he never left you, but this is what you imagine all the time.
4) Now imagine what you would do as this imagination warps your memory, making you think that this is actually happening but it is not. It's just in your head.
It doesn't matter if the man is already your boyfriend or is just someone you admire. To a yandere, the target is already theirs. This ability (or tendency) to impose fantasy into reality is what defines the yandere. Where The Stalker is just someone who wants to make someone they admire theirs, The Yandere already have that someone theirs in their minds.