Caroline: Are you in love?
Mary: Well, I... I do love him, I suppose. Not quite like when we first met. I trust him, really. He's my closest friend. But, what do you mean by in-love?
Caroline: I mean that you'd do absolutely anything for the other person, and you'd let them do absolutely anything to you. Anything.
Mary: Anything?
Robert: Let me tell you something: My father was a very big man. And all his life he wore a black mustache. When it was no longer black, he used a small brush, such as ladies use for their eyes. Mascara.
Answer: While it's possible that they slept in separate beds because of Caroline's chronic back pain, and Robert could accidentally hurt her in the night by moving, it's more likely that it's a sexual kink of sorts, a way to 'deprive' them both of each other. They murdered Colin because Robert was highly unstable (Caroline was utterly dependent on him and was also unstable as a result) and since they enjoyed pain during sex, it's implied that they thought it would be a great sexual thrill to seriously hurt someone else, which would explain why they appeared amorous with each other as soon as Robert cut Colin's throat.
Purple_Girl