[T]he world is governed by demons and that he who lets himself in for politics, that is, for power and force as means, contracts with diabolical powers and for his action it is not true that good can follow only from good and evil only from evil, but that often the opposite is true. Anyone who fails to see this is, indeed, a political infant.
More broadly, Weber argues that there are broadly speaking two types of rules for ethical conduct: an "ethic of ultimate ends" and an "ethic of responsibility." These both have a role to play, and exist in tension with each other. But Weber seems to emphasize that even when focused on ultimate ends like justice or fairness, one still has some responsibility to take account of the likely and foreseeable results of given actions. Weber writes:
[A]ll ethically oriented conduct may be guided by one of two fundamentally differing and irreconcilably opposed maxims: conduct can be oriented to an 'ethic of ultimate ends' or to an 'ethic of responsibility.' This is not to say that an ethic of ultimate ends is identical with irresponsibility, or that an ethic of responsibility is identical with unprincipled opportunism. Naturally nobody says that. However, there is an abysmal contrast between conduct that follows the maxim of an ethic of ultimate ends--that is, in religious terms, 'The Christian does rightly and leaves the results with the Lord'--and conduct that follows the maxim of an ethic of responsibility, in which case one has to give an account of the foreseeable results of one's action.
For context, here's a fuller quotation from the "the world is governed by demons ..." passage.
My colleague, Mr. F. W. Forster, whom personally I highly esteem for his undoubted sincerity, but whom I reject unreservedly as a politician, believes it is possible to get around this difficulty by the simple thesis: 'from good comes only good; but from evil only evil follows.' In that case this whole complex of questions would not exist. But it is rather astonishing that such a thesis could come to light two thousand five hundred years after the Upanishads. Not only the whole course of world history, but every frank examination of everyday experience points to the very opposite. The development of religions all over the world is determined by the fact that the opposite is true. The age-old problem of theodicy consists of the very question of how it is that a power which is said to be at once omnipotent and kind could have created such an irrational world of undeserved suffering, unpunished injustice, and hopeless stupidity. Either this power is not omnipotent or not kind, or, entirely different principles of compensation and reward govern our life--principles we may interpret metaphysically, or even principles that forever escape our comprehension This problem--the experience of the irrationality of the world--has been the driving force of all religious evolution. The Indian doctrine of karma, Persian dualism, the doctrine of original sin, predestination and the deus absconditus, all these have grown out of this experience. Also the early Christians knew full well the world is governed by demons and that he who lets himself in for politics, that is, for power and force as means, contracts with diabolical powers and for his action it is not true that good can follow only from good and evil only from evil, but that often the opposite is true. Anyone who fails to see this is, indeed, a political infant.
(For those not familiar with the term, the Upanishads are an ancient Sanskrit text, written from about 800 BC to 200 BC, that laid out some of the central principles of Hinduism.)
And here a fuller quotation for the ethic of ultimate ends and ethic of responsibility passage:
We must be clear about the fact that all ethically oriented conduct may be guided by one of two fundamentally differing and irreconcilably opposed maxims: conduct can be oriented to an 'ethic of ultimate ends' or to an 'ethic of responsibility.' This is not to say that an ethic of ultimate ends is identical with irresponsibility, or that an ethic of responsibility is identical with unprincipled opportunism. Naturally nobody says that. However, there is an abysmal contrast between conduct that follows the maxim of an ethic of ultimate ends--that is, in religious terms, 'The Christian does rightly and leaves the results with the Lord'--and conduct that follows the maxim of an ethic of responsibility, in which case one has to give an account of the foreseeable results of one's action. You may demonstrate to a convinced syndicalist, believing in an ethic of ultimate ends, that his action will result in increasing the opportunities of reaction, in increasing the oppression of his class, and obstructing its ascent--and you will not make the slightest impression upon him. If an action of good intent leads to bad results, then, in the actor's eyes, not he but the world, or the stupidity of other men, or God's will who made them thus, is responsible for the evil. However a man who believes in an ethic of responsibility takes account of precisely the average deficiencies of people; as Fichte has correctly said, he does not even have the right to presuppose their goodness and perfection. He does not feel in a position to burden others with the results of his own actions so far as he was able to foresee them; he will say: these results are ascribed to my action. The believer in an ethic of ultimate ends feels 'responsible' only for seeing to it that the flame of pure intentions is not quenched: for example, the flame of protesting against the injustice of the social order.