What if Rain were to actually die in his hands?
Perchance, the Prince doesn't lose control and decides to spare her, he wouldn't even be able to, because the poison would completely defeat his stable frame of mind to constraint his wildness.
There was one more thing Flora hadn't disclosed to Vulcan.
That was, the after effects of the nightshade; the rapid heartbeats, a blurred vision, and then the hallucinations...
The mighty Prince of death might not be able to resist these effects, thus, he would likely not be sane enough to show mercy, even if he miraculously wishes to.
*
"Is Prince Derek not returning tonight?" Rain questioned Lucas impatiently for the umpteenth time that evening.
Unfortunately, Lucas had the same disheartening answer; "I can't be sure, My Lady. Its getting cold and late, I think you should retire to your chamber. I'll bring you a brazier."