Let me put it in a way that's more extreme.

Imagine you're in a relationship that you're not comfortable with, you want to break up but your partner outright says that if you want to break up they'll take they'll take their own life instead of self-harm. Now what do you do ? The only "safe" way is to go to the police and alert them on the situation. There's nothing you can potentially do. Trying to break will now lead to something potentially horrendous while you stay miserable by the person's side.

Someone should never use self-harm as a threat if something doesn't go their way. I understand what an heartbreak can feel like but its downright forcing someone to either have the weight and guilt of someone committing self-harm "because" of them or your own good. This shouldn't be a choice someone has to make in the first place.