A sick man's trick.

I am glad it's not supernatural or magical, he was only using poison to kill them or make them commit suicide. Ever since Diana said that he was laughing when he burnt his house and got all the insurance money from his parents, I knew that the grim reaper was actually a sick person who likes to kill people for his own ill satisfaction.
Anyway, I still think that this is the most interesting case so far. Hopefully, the upcoming cases will be equally captivating...