When Marcus persists in seeking out Roo's help, Roo sees her chance to use the situation to her advantage. If she helps Marcus, he would owe her, meaning she could finally put an end to her curse.
In her quest to get Marcus to heal her, Roo resorts to extreme and unconventional means involving banana ice cream and chicken soup. She enlists the help of EJ, but her attempts are ultimately thwarted.
When Roo decides the only way forward is for her to have sex with Marcus, EJ is less than on board. Wearing her best supportive face, EJ questions Roo's thinking. What if instead of Marcus healing Roo, Roo ends up killing Marcus? Is that a risk Roo is willing to take? It's not.