Adding some emotional turmoil for your yandere will make it more believable and somewhat more interesting. Of course, there are generically insane people out there in the world that feel no guilt or remorse..... But, your yandere isn't insane. They're in love. They want to protect what matters the most to them in the entire world~ your main character.
A chapter where your yandere openly expresses sorrow over they've done will humanise them and earn them sympathy from the reader as well as the MC.
You can pick the duration of the honeymoon phase.
One week? Three years?
Just remember.... All good things must come to an end.
ALSO, REMEMBER THAT THERE IS A HUGE DIFFERENCE BETWEEN A YANDERE AND AN ABUSIVE SIGNIFICANT OTHER!!!!