Most evil killer(s)?
As the question asks who were the most evil killers on the show? Here are my 3 top picks.
The ABC Murders
To kill 3 people just to throw everyone off the real motive is the work of a true evil soul
Cat Among the Pigeons
Sure one of the victims had it coming to them but the killer was a real nasty piece of work
Murder in Mesopotamia
So the killer may be unbalanced but that is no excuse for killing people because of your own fears.