When you go in with a mission and that mission is accomplished, you leave feeling satisfied.The fact that they believe the killer may have filmed the act on their cellphone tells me they have quite a bit more video with many more clues. I think the Perp probably left the scene just as calmly as they entered. very twisted indeed.
I have the feeling the person thinks/thought they did it exactly as planned, and would/has gotten away with it.
But if you take a video of it, you're either wanting to relive it or prove that it has been done.
MOO