You are going to get caught with that attitude silly! You need to be more subtle about it if you are going to kill your mother-in-law. For example use one of these gift ideas for your mother-in-law thar appear to be genuine gifts but might accidentally kill her: http://whattogetyou.com/mother-in-law-gift-ideas.html
Now you can make it look like an accident (or at least have a laugh at these ridiculous ideas)