Here is an example of Utilitarianism.
Suppose, to save the life of a man it is necessary to cut off the hand of another man?
This seems like a simple and moral trade-off. Certainly, to save a life the sacrifice of another man's hand is reasonable. But, what if it took the hands of 10 men? What if we needed the hands of 100 men? Now things start to be a little trickier.
You might say this is an unfair example but we make these choices all the time.
How about, for 300 million Americans to have cheap gasoline 100,000 innocent Iraqis must die? Is that reasonable?
For one man to be a billionaire, one million people must live in poverty?
For one man to be healthy, one thousand people must be sick?
We make these choices every day and have no problem with trading the suffering of some for the happiness of others.