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They will hurt themselves to show their true devotion to their love interest or to make it clear to anyone that they belong to their love interest alone. Their self-inflicted wounds and scars will represent their infinite and loyal love for their love interest, treating their own body like a canvas created just for the admiration of the love interest and a representation of their love towards them.
Their self-harm behavior will be used to prove to their love interest that they would do anything for their love interest. They don't care if they themselves get hurt or end up dead, so they show this in front of the love interest inflicting that damage on themselves directly by their own hands.
Shock trope:
If they see that they are not enough for their love interest or they see themselves having any physical "defects," they are also willing to modify their own body in order to become the ideal partner for the love interest, even if this heavily damages them forever.
They can also try to get their love interest's attention by harming themselves. They will hurt themselves to gain something from the love interest. They manipulate them by threatening more harm to themselves if their love interest doesn't do what they want their love interest to do.
But it is never the intent of self-harm yandere characters to hurt themselves. They're interested in love, not harming themselves. It is the lack of devotion, whether circumstantial or intentional, from their love interest, and the subsequent emotional pain, that leads them to harming themselves.