In Major Barbara, what is Andrew Undershaft's justification for selling arms and munitions to any buyer without discrimination?
George Bernard Shaw's 1905 play Major Barbara is partly satirical in its attack on weapon sales, and partly serious in its depiction of business as something outside the realm of ethics. Andrew Undershaft, the titular Barbara's father, is a weapons manufacturer and salesman who sells to any buyer, and is proud of his business success.
One of the play's major themes is the relative nature of "doing good" vs. "doing evil." Barbara's work for the Salvation Army is altruistic in nature, but it has little real-world effect on the people it helps; they accept charity and return day after day, never seeking to rise above their poverty. Meanwhile, while Undershaft sells weapons to all buyers, including those who would use them for evil purposes, his employees are all comfortable, well-paid, and happy:
UNDERSHAFT. Cleanliness and respectability do not need justification, Barbara: they justify themselves. I see no darkness here, no dreadfulness. In your Salvation shelter I saw poverty, misery, cold and hunger. You gave them bread and treacle and dreams of heaven. I give from thirty shillings a week to twelve thousand a year. They find their own dreams; but I look after the drainage.
Poverty and slavery have stood up for centuries to your sermons and leading articles: they will not stand up to my machine guns. Don't preach at them: don't reason with them. Kill them.
(Shaw, Major Barbara, eNotes eText)
This sort of thinking would surface later in Ayn Rand's philosophy of Objectivism, but here it stands as justification for sales. Undershaft is not concerned with the actions of others, only his own actions, and to his mind, he has improved the world. Perhaps he is only improving it to a small extent, and perhaps others will use his weapons to degrade it, but he is satisfied that he cannot control what other people do, only what he can himself accomplish. By destroying poverty in his small influence, he can spread the "good" of pride in work and pride in self-respect. The poor have no self-respect, so they do nothing to improve themselves; his workers have self-respect, and so they can "find their own dreams" in their own time.
Of course, he does not sell exclusively to evil men, and so he likely reasons that enough good men buy his weapons to offset the evil; in a classic 2nd Amendment situation, Undershaft would arm everyone and let the good people defend themselves, since in his mind, the good outnumber the bad.