The only way to get that scene is to outright refuse to kill Isobel while Sceleritas Fel is talking to you when everyone is asleep.
In one of my playthroughs I made the mistake of saying one of these choices, though it was probably the first one;
- "I felt the desire to kill her the moment we met..."
- "What has she done to deserve such a fate?"
Hoping it'll open some further interesting dialogue with him to explain the urges, but the dialogue instead ended and the game took this as Dark Urge being open-minded towards the idea of killing her. So I never got the romantic murder scene at all, despite refusing to kill Isobel when talking to her.
Also the scene is exclusive to ACT II I believe, so your only choice is to reload prior to talking to him and outright saying NO to his face. He'll become disappointed, once you refuse Isobel the journal objective will state that you're acting weird and you'll get the cutscene with your romance instead the next long rest.