Although some claim that
Utilitarianism can be used to justify actions that commonsense morality tells us are wrong when more factors are taken into account, utilitarianism conforms to
conventional morality.
In the example of a single person being killed in order to save hundreds, society wouldn't just be trading one person's life for the lives of many others - it would also be forcing all citizens to live with the possibility of being sacrificed for the greater good at any time. That would be a huge burden, and would cancel much of the gain from the sacrifice. So for example, killing one person to save two probably wouldn't be worth it, because it would cast fear on the entire population and only gain one life in exchange. But, killing volunteers to save the country would be worth it, and is in fact done in the military.
In the case of the Peeping Tom who creates goodness (pleasure) by spying on his neighbor, while not causing any perceptible harm, there still must be a possibility that he will get caught, causing very bad feelings for the woman, to know she'd been spied on the whole time, and probably bother her for the rest of her life. This potential harm is so great that it balances out the benefit of peeping. Similarly, if he increased the benefit by spreading the pictures on the internet, it would also increase the potential harm to her, as well as her chance of finding out, making the whole proposition a loss for society on average.
The potential for damage to the peeped is also why looking at 100 year old peeping tom photos is not wrong, while taking them now is.