"The Good of the Many Outweighs the Good of the Few"
Okay, it's a clichéd line from Star Trek, but it neatly sums up the essence of Moral Deontism.
In one sense, Moral Deontism can be seen as the opposite of Moral Absolutism - the doctrine that an individual must not commit an immoral act regardless of the situation. A Moral Deontist would be prepared to take the life of another human being if it meant saving the lives of many others, whereas a Moral Absolutist would take the approach "It is wrong for me to kill a person. It is not my fault if others die as a result - I am not prepared to commit an immoral act."
The problem of Moral Absolutism is, in this example, that although the people who die would not have been killed by the Moral Absolutist, it was within the power of the Moral Absolutist to have prevented the deaths of others by comitting an act of immorality. The Moral Deontist would be prepared to commit this act of immorality in order to further the "greater good".