Curses are rarely used to kill, apart from the morality of the event most practitioners simply lack the personal power necessary to see the curse through and kill someone. Potions on the other hand have a historical reference but they're not subtle things and I can almost garunteed that using it on anyone (including yourself) would be found out and is very much illegal, what you're basically asking for is a poison, almost all of which have been traced biochemically and we now know the causes if not the treatment for them. This type of potion whilst having magical origins is rooted very much in science.
You said this as it was everyone's belief, maybe not literally, but that's what we get from you post, which is the reason witchykid felt like it was necessary to notify that not everyone believes in it. And it was certainly not superfluous!