What would you do if this were real life?

I can think of a few possibilities:

• Think about if the character is evil, despite your love. Can you come to terms with the fact they need killing? Think about the ending to Of Mice And Men.
• Can you alert the authorities? If the players want blood, then alerting the local militia of their plans would put them on the hook for murder. They may then go for capture and fair trial, which is a much nicer option.
• Can you alert the evil character to the player's intentions so that they can flee?
• Can you explain the fact that you will die before you let them kill her to the party/GM? They may act differently, or the GM may engineer a solution that doesn't result in your collateral death.

It sounds like a puzzle to me that you need to try and solve. But you don't always win.

What would you do if this were real life?

I can think of a few possibilities:

• Think about if the character is evil, despite your love. Can you come to terms with the fact they need killing? Think about the ending to Of Mice And Men.
• Can you alert the authorities? If the players want blood, then alerting the local militia of their plans would put them on the hook for murder. They may then go for capture and fair trial, which is a much nicer option.
• Can you alert the evil character to the player's intentions so that they can flee?
• Can you explain the fact that you will die before you let them kill her to the party/GM? They may act differently, or the GM may engineer a solution that doesn't result in your collateral death.

It sounds like a puzzle to me that you need to try and solve. But you don't always win.