Sabotaging Rebecca's still results in her refusing to speak to the Chosen One again, and triggers a negative ending for The Den, either making Metzger and the slavers the dominant power in the area, or if he has been killed causing The Den to disperse due to the destruction of its economic powers.
Rebecca is the "good" leader of the Den. As such, she will also reward the player character for killing Metzger.
Rebecca will only give quests to a good Karma Chosen One.
Aa female character named Buffy can talk to Rebecca and she'll give her a metal armor, some stimpaks, an SMG and ammo, some grenades, and a flower. These are all incredibly good starting items, or can be traded to the local merchants for different equipment. The conversation will only happen if the player character is wearing at least a leather armor.