The killing of Michael Brown by a cop in Ferguson, Missouri, on Aug. 9 and the nights of sometimes violent clashes between protesters and police that followed were troubling to me for two very different reasons. The first is obvious: it's disturbing to see such stark evidence of the persistence of racism and police brutality in American life. The second focused less on the events themselves than on the reaction to them on social media, especially Twitter.

Night after night for a week and a half, as cops armed with military gear pointed heavy weaponry and fired tear gas at civilians, a slew of liberal activists, journalists, professors, and amateur commentators whipped themselves into a frenzy of moral indignation, unleashing a flurry of angry tweets — against the cops themselves, against Ferguson police chief Tom Jackson, against Ferguson Mayor James Knowles, against Missouri Gov. Jay Nixon, against white America, against Barack Obama.

I get it. Those were infuriating nights. I understood and to some extent joined in the anger. I even wrote an impassioned essay of my own once the unrest had subsided. It joins a long list of columns in which I've gleefully played the moralist, evaluating, judging, and even denouncing the evils of others. I've done this with Israel, the wealthy, the GOP (on many occasions), and the white American majority on racial issues.

But there is another, contrary theme in my writing: the dangers of moralism. This comes out most clearly in my writing on foreign policy (where the temptation of policymakers to allow themselves to be guided by abstract moral imperatives often leads the country into peril). The theme also animates my controversial columns on the overly zealous reaction of some liberals to religious traditionalists — whether in their continued rejection of the legitimacy of gay marriage or in their effort to avoid having to pay for the purchase of certain forms of their employees' birth control.

The reason for my ambivalence about moral passion has its roots in convictions about the pluralistic character of moral truth — and a form of self-deception to which liberals are peculiarly prone.

On the first point, my guide is historian and philosopher Isaiah Berlin.

Berlin is famous for having criticized the "monism" of many moral theories and religious doctrines. By insisting that moral truth is simple, singular, and easily graspable by all who genuinely seek to know it, monists both distort the true, complex character of morality and encourage the intolerant fanaticism of the true believer. In Berlin's view, monistic thinking marks Platonism, certain fervent forms of religious piety, and the most rationalistic strands of Enlightenment thought. It also inspired the totalitarian political systems that turned the 20th century into a slaughterhouse. All of them were examples of monistic moralism run amok.

In place of monism, Berlin substituted moral pluralism. While many critics have contended that Berlin's pluralism is indistinguishable from relativism, Berlin himself always denied the charge. Pluralism affirms that there are objective moral truths and intrinsically valuable ends, he insisted, but it recognizes that these truths and ends clash with one another. Liberty is an objectively worthwhile moral-political ideal, for example, but so is equality — and there are countless examples of these two values coming into irresolvable conflict with each other.

For Berlin, the unavoidability of conflict among objectively valuable moral ideals gives human life and politics a deeply tragic cast, with trade-offs and unsatisfying compromises often the best that we can hope to accomplish. The ineradicable pluralism of morality also makes liberalism the most humane political arrangement on offer, since (in theory at least) it allows people with very different notions of what constitutes the highest human good to live together in relative peace, despite their differences.

In Berlin's reading of the past, monists are history's greatest troublemakers — because, in blatant rejection of the moral facts, they deny the need for tragic trade-offs, insisting instead that just one set of values (their own) is worthwhile. It's hardly surprising that this moral conviction tends, paradoxically, to inspire great acts of immorality — with one moral ideal often violently transgressed in the name of another. The fact of pluralism shows that what we need, instead, is moral toleration.

This was a message developed in a slightly different way by literary critic Lionel Trilling, who turned to literature as an antidote to the sometimes dogmatic, self-satisfied moralism of his fellow postwar American liberals. From the complexity of the greatest literary characters, Trilling derived a potent lesson about moral motivation. Even a person who seems to have the purest intent nonetheless acts partially out of self-interest — to appear admirable to peers, to receive praise from like-minded comrades, to demonstrate his own righteous superiority to those on the other side of a conflict.

All moralists have mixed motives, but liberal moralists are distinctive in going out of their way to deny them. That makes liberals prone to self-congratulation, quick to flatter themselves for their own good intentions, and all too eager to dismiss as "reactionary" anyone who holds a different moral view. "When the liberal intellectual thinks of himself," Trilling suggested, "he thinks chiefly of his own good will and prefers not to know that the good will generates its own problems, that the love of humanity has its vices."

This self-righteous attitude is a problem because, as Adam Kirsch writes in his essential study of Trilling's thought, it convinces liberals that they need not concern themselves "with all the ways good can produce bad, through unintended consequences, or unacknowledged motives, or fanatical zeal."

The lesson for self-reflective liberals is plain. Go ahead and express outrage at a potentially murderous and racist abuse of power. By all means object strongly to a Supreme Court ruling that curtails rights you passionately believe in. But do so thoughtfully, alive to the inevitability of moral conflict and the unavoidability of mixed motives.

Otherwise liberals risk reproducing the same moral vices they so ardently denounce in others.