Although it is not expressly stated in the umbrella values of most dogmas or individual moral compasses, eating people is generally frowned upon. Most would say, “it goes without saying.” So, morally, we have proponents that say cannibalism is bad.
To look at it from a different metric of good and bad, the United States Department of Agriculture prefers lean proteins to fatty red meats, which will overburden your daily limit of empty calories. We Americans, bursting on a culture of hamburgers and thick steaks, consume large quantities of empty calories that create a decidedly full figure. Therefore, nutritionally, cannibalism is not the diet for beach season.
There exists a third reason (as well as many more that will not be addressed here) why cannibalism is bad for you, and it seems like karma. If you eat human brains, you may contract a disease that will return the favor: kuru will eat your brain. Worse yet, there is no cure.

Kuru may be untreatable, but it is also rare. So rare in fact, that it is limited geographically and temporally to the Fore tribe in Papua New Guinea, who experienced a sustained epidemic which peaked during the 1950s and slowly declined over the ensuing decades. At its peak, the disease was the most common cause of death of Fore women. The Fore tribe consumed their dead not in a malicious fashion, but as a funeral rite. After scientists identified the disease, cannibalism effectively ceased in the early 1960s – for kuru, the best treatment is prevention.