Wouldn't be so good if someone just took a wrong turn! Were you envisaging a conversation first? Maybe the target would wear something agreed upon. A target perhaps? Sorry- my gallows humour.
I know you're not serious but if you were, trouble is- they may still class it murder. They'll still say people who want to die are 'vulnerable' and being taken advantage of by sadistic homicidal maniacs. There was a case- maybe don't look at the details- it's EXTREMELY NSFW but in Germany, a sadistic killer advertised for a victim, got one, killed him and got convicted for murder... and other things.
I'm also not too sure that killers necessarily want willing victims. I imagine it's more the thrill and power of killing someone who doesn't want to die. Not that I want to really imagine what it's like to want that.