Waiting on Wednesday

Her Dark Curiosity by Megan Shepherd

Months have passed since Juliet Moreau returned to civilization after escaping her father's island—and the secrets she left behind. Now, back in London once more, she is rebuilding the life she once knew and trying to forget Dr. Moreau’s horrific legacy—though someone, or something, hasn’t forgotten her.
As people close to Juliet fall victim one by one to a murderer who leaves a macabre calling card of three clawlike slashes, Juliet fears one of her father’s creations may have also escaped the island. She is determined to find the killer before Scotland Yard does, though it means awakening sides of herself she had thought long banished, and facing loves from her past she never expected to see again.
As Juliet strives to stop a killer while searching for a serum to cure her own worsening illness, she finds herself once more in the midst of a world of scandal and danger. Her heart torn in two, past bubbling to the surface, life threatened by an obsessive killer—Juliet will be lucky to escape alive.
With inspiration from Robert Louis Stevenson’s The Strange Case of Dr. Jekyll and Mr. Hyde, this is a tantalizing mystery about the hidden natures of those we love and how far we’ll go to save them from themselves.

The ending of Madman's Daughter. THE ENDING. OMG. THAT WAS THE ABSOLUTE WORST SOMEONE SHOOT ME. 
Not to mention, you guys! Jekyll and Hyde! And a murderer. And woah there. "facing loves from her past" ARE WE TALKING ABOUT WHO I THINK WE'RE TALKING ABOUT? :D