How to Prevent Party from Killing/Capturing an important NPC

Newish DM here looking for advice on how to avoid the party from capturing or killing an important NPC. One of the characters in the party of the campaign that im running was a survivor of a massacre, and the NPC is the only other survivor of that massacre who thinks the PC is hunting her down (they are not). Long story short, this character is important to a later section of this character's quest and they are supposed to run into each other a few times between now and then. However, the party doesn't seem keen to take any prisoners and they've already tried to kill the character once. Does anyone have any advice on how to prevent the party from killing this NPC? The next encounter they will see her is in a forest where she would avoid direct conflict with the party, but they seem to resort to attacking before anything else lol.