Help with the antagonist in my story?

You could make Demelza like Isla, but she wants the power so badly so she's willing to kill her to get what she wants. That way, you can tell how dangerous she is since she'll do anything to gain power. You could also make the assasination attempt to kill Isla longer, drawn out, and with many parts to the plan. Then you can give Demelza something to focus on during the story. A character's personality is also an indication of whether they are good or evil. So her thought process could clue readers in to her true self. I hope this helps.