I quote at length from Calvin's sermon on Galatians 6:10 on why "we must endeavor to do good to all men, yea even to such as are unworthy, even though they were our deadly enemies":

"Now when he saith to all men, it is to show us that although men discourage us to do them good, yet we must not cease to do still as God commandeth us. For (as I have touched already) we must not look what every man is, nor what he deserveth: but we must mount up higher and consider that God hath set us in this world to the end we should be united and knit together: and that forasmuch as he hath imprinted his image in us, and we have all one common nature: the same ought to move us to succor one another. For he that will exempt himself from relieving his neighbors, must get him a new shape, and show that he intendeth to be no more a man: for so long as we be of mankind, we cannot but behold our own face as it were in a glass, in the person that is poor and despised, which is not able to hold out any longer, but lieth groaning under his burden, yea though he were the furthest stranger in the world. Let a Moor or a Barbarian come among us, and yet inasmuch as he is a man, he bringeth with him a looking glass, wherein we may see that he is our brother and neighbor. For we cannot abolish the order of nature, which God hath set to be inviolable. So then we be bound to all men without difference, because we be all one flesh, as the Prophet Esay avoweth, saying: (Isaiah 58:7) Thou shalt not despise thine own flesh. As if he should say, they that are niggardly [stingy, covetous] and pinching, and shrink away when they should do good, do not only despise God, and reject his word: but also are ugly monsters, because they consider not that there ought to be a community among all men. Thus ye see why Saint Paul saith expressly, that we must endeavor to do good to all men, yea even to such as are unworthy, even though they were our deadly enemies. Truly this is hard, and contrary to our inclination: but yet therein God trieth us so much the better. For if we do good to such as deserve it, or to such as are able to recompense it: it is no declaration or proof that we be willing to serve God: for it may be that we had respect to our own profit." (pages 588-589)