What does this quotation tell us about the hero in The Deerslayer? "I hold it to be onlawful to take the life of a man, except in open and generous warfare".
This quotation gives us a sense of the moral convictions of the speaker, the hero of the story, Natty Bumpo. Although he is by no means a complete pacifist, he has a great respect for life. To him, it is wrong to kill a man, unless it is under the conditions of warfare. Although he would not hesitate, in fair and "open and generous" war, to defend himself and those dear to him, he would not ever wantonly take another man's life without this justification.
Natty Bumpo is very "clear in his mind" about what is right and wrong, and what is required of him as "a Christian man". Unlike his friends, whose values are not as clear-cut, he refuses to take part in the practice of scalping, explaining that, while doing so may be lawful for Indians, it is not acceptable for the white man. In adhering unwaveringly to his own self-imposed principles, he does not condemn others for behaving differently, accepting as true and natural the reality that different groups have different customs and standards.
Natty Bumpo is a brave man, but he will kill only when he believes "it is lawful and the situation demands it". His respect for life extends even to the animal world. He kills deer only when he has need of their meat or hides, and in the one incident when he momentarily forgets his principles and shoots an eagle "for the fun of it", he is bitterly repentant.
Supporting quotes from http://www.enotes.com/deerslayer/characters