In economics, a consumer's utility is defined as the amount of satisfaction a consumer gets from the consumption of a particular good or service. It is certainly possible to get "too much" utility from a given good or service.
Consider, for example, an all-you-can-eat buffet restaurant. When you go to such a place, you want to get the most out of your money, right? But what happens if you go overboard?
In that case, you are getting so much utility from your good (the buffet you paid for) that you overdo it and get sick. At that point, your "good" has become a "bad."
In real life it is possible to make a mistake and choose to buy and consume a commodity that does you more harm then good. The world is full of such examples - like drugs, alcoholism, and obesity due to junk food. However, the theoretical concept of utility and diminishing marginal utility in economics does not takes these into accounts. Economics assumes that individual are able to judge the utility of all goods and services available to them and take right decision based on that. Under theses assumptions of economics it is possible for people to get so much utility from a good that its marginal utility becomes zero, but not negative. The most common example of a good that is consumed so much that its marginal utility turns zero is the air we breathe. The overall utility of air for breathing is very high. But once we have enough air to breathe, its marginal utility is zero.