This is perhaps the hardest question of all, and I have no idea what the answer is!
Different proposals have been made by moral philosophers regarding how to reply to this question. First of all, there is disagreement regarding the domain of moral theories: should we assess actions or should we assess people? However, for the purposes of replying to this question, I will focus on the former – the assessment of actions. There is a clear divide between two traditions in moral philosophy: those who are consequentialists and support that the value of an action is determined by the consequences it brings about, and those who are deontologists and support the claim that there are actions that are good and bad in themselves, independently from the consequences they bring about.
Consequentialism makes the measurement of the value of actions more systematic; however, one faces other problems, such as one might end up adopting an attitude that is too mechanical. I once became obsessed with maximising the amount of good things the money I donated to charity could do – and every face in the street that asked for money became a calculation to me. One day, a woman asked my mother for some money to buy a chicken for her children. In my head, I was thinking: ‘oh, that same money would have a much more efficient use if given to x or y,’ but then I saw the look in my mother’s face: it was the look of a mother, who sees another mother in need, and simply doesn’t question the intuition that the right thing to do is to help this woman.
As I said at the beginning, I don’t know the answer to this question. If anything, what we can conclude is that we must be careful when judging someone. In fact, one suggestion that I particularly like is the one given by Adam Smith in The Theory of Moral Sentiments. He defends that we should use empathy when assessing whether or not we think a given action is morally bad. We should try to put ourselves in the shoes of the person who performed the action, as described in the following quote:
As we have no immediate experience of what other men feel, we can form no idea of the manner in which they are affected, but by conceiving what we ourselves should feel in the like situation [. . . ] [I]t is by the imagination only that we can form any conception of what are his sensations [. . ..] By the imagination we place ourselves in his situation, we conceive ourselves enduring all the same torments, we enter as it were into his body, and become in some measure the same person with him, and thence form some idea of his sensation, and even feel something which, though weaker in degree, is not altogether unlike them. [. . ..] That this is the source of our fellow-feeling for the misery of others, that it is by changing places in fancy with the sufferer, that we come either to conceive or to be affected by what he feels, may be demonstrated by many obvious observations . . . (Adam Smith, 1759: 47-8)
I love this attitude. I believe it would make the world a better place, however that might be defined or quantified.