I've been wondering if the other person that has to die according to Slade won't be Laurel but instead Felicity. Felicity pushed him to stop killing. She softens him. She is in a way his light. With Laurel, there's a lot of angst, drama and dirty laundry there. Someone like Felicity actually brings a calmness to him. Plus the montage in the episode Slade came back, he said the "corrupt those he loves" and they showed Felicity and the other two shown, Det. Lance and Roy, have already suffered in the hands of Slade. This leaves her as the one character shown in that montage to still not be affected.