The odds of any given person dying is over 1 in 6,800,000,000. The population is in a state of growth. At any time somewhere on the world there is someone dying, at every moment in time. Even if you could prove the button was indeed killing someone, pressing it twice a day would be more than enough money, and would mean that the population would still be fine for you to kill loads, and still not have the economy collapse on you making your $100 per dead head worthless.
If I was seriously given this option I would spend a while thinking about it, and only use it in desperation. Although anyone alive can die at any given time (heck, it's possible the entire planet could be destroyed at any given second, and we'd have no warning), taking people before there time could still have major repercussions, even if it was people you never personally knew yourself. What if the first death was the guy who ultimately would've lived long enough to be your one true love, or someone who someday was destined to save your life? The chaos theory application of what could be attached to a button that can kill any random person for money makes each press a very tough decision, even if you're desperate for money.
