Suppose, a strange object flies over a city and drops a box with a button on it, which someone picks it up. Upon picking up the box, it telepathically reveals to him that every time he presses the button, a random individual in the world instantly dies, but whomever uses the box or is holding any box like it is not at risk of this phenomenon.
The box also reveals to him how to create more boxes like it. Suppose the complexity and intricacy of the box is similar to modern consumer electronics, but that it has a magical property that only those who touch a box and thus receive its knowledge can create one.
Given this, what would be the fastest and most effective way to eradicate humanity.
Since many answerers were asking how much time it would take to create one of these boxes, let's assume that creating one of these boxes would consume a total of 30 hours of labour, that could be split up over several days. This can be shortened to 20 hours if two people are working on the same box, but any additional people would not be able to help substantially.
Also, to give an idea of the size of the object I envision, the box is large enough that one can just barely hold it firmly in the palm of their hand. To clarify, the person who presses the button, in addition to anyone who is holding any death-box, whether it is the original or a copy, is safe from being killed by the button. One must have physical contact with a box in order to be considered "holding" that box. An animal or mechanism can be used to press the button, but remember that no one is safe unless they are holding a box.