"I think when it came to our arrangement," Luna said, taking the new drink and putting the last of her wine aside, "it was more a problem of the who than the what."
Though, to be honest, she did know Will had a problem with what she did in general, even the parts that weren't with Merlin. But Will was a folklore hero: shades of grey were different to them than they were to a regular questionable human. Luna knew that she had never been (and would never be) anyone someone would call a hero. Despite all her good intentions, she was too grey to ever be truly good. Luna had come to accept that.
Perhaps that was why she was drawn to the people she was: Peitho, the dangerous seductress. Hermes, the trickster thief. Merlin, the ambiguous wizard. Perhaps it was why she and Will had fallen apart, why there would always be a distance between them that could only be filled with longing and desire. (Because she still wanted Will, but their friendship now felt so delicate, so ready to be lost again. She wasn't sure they would ever fully repair from what had happened between them.)
Tinsel wasn't grey, but she was human, just like Luna, despite who her parents were. Luna knew she would throw herself in front of a train to protect Tinsel, but she also worried that her own grey was going to leave marks that might not come out. She'd already forced Tinsel to see someone be brutally murdered - what else might come? They were both grown women, but Luna felt such a need to protect Tinsel.
"I can see why he'd feel the drive to ride in on a white horse and rescue Elsie."