When it comes to a chunk of sugar as delicious as a KitKat, all they really need to do is plant the word "KitKat" in your brain and you'll want one. Or maybe it's just me.
Start here : http://en.m.wikipedia.org/wiki/Nestlé_boycott
Certification isn't perfect, and different labels focus on different things (though most cover child labor, environment, fair working conditions), but they give a signal to the corporations that you are expecting more than just sugar at the lowest price. Some Nestle products in some markets have third-party certification. If you want to buy Nestle, buy those.