I am not sure I would go so far as to say that "the love of theory is the root of all evil" is the best sentence ever, but it does open an arena of thought in which to explore the inexplicable: why are intellectuals always so willing to press on against all factual, physical evidence when their beloved theories don't work, are demonstrably false, even when they are so disastrous that they result in the deaths of millions? As I have wondered here before, if you really wanted the best for people through the implementation of your theory (i.e., Marx's communism, Mao's cultural revolution), wouldn't you start to question your conclusions when you reached, say, 20 million deaths of your own countrymen...? I mean, wouldn't you start to get a little doubtful around 15 million or so...give or take a few mil? How many bodies would it take for a you to step back and say, "Whoa...this can't be right."
There are men who develop theories and when the theories are proven wrong, they adapt to the new evidence and change or abandon their theory. They can do this because their ego isn't aligned with being right; their ego is aligned with finding truth. This is quite different. Men who become completely consumed with being right will destroy anything or anyone rather than admit they are mistaken. They are certain men; they are dangerous men.
My belief is that it is not the love of theory that makes men mad. It is the investment of their ego in the theory. It is ego inflated by advanced intellect that creates the pure evil of a personality willing to impose its vision - its certainty - on others, that drives men to push through all restraints of conscience.
This is a madness to which the left, the progressive, the modern liberal is particularly susceptible, although its allure is enticing for all men. The modern liberal believes himself above the nonsense of superstition, the irrationality of religion, the uneducated assumptions of the common man with his common sense. His entire sense of self and his view of the world is defined by the ideas of men of theory who are often invested in being right rather than in finding truth. For truth is elusive. But with enough power, you can always be right. Having spurned religion, the liberal sees only superstition instead of wonder, ignorance instead of the humble acknowledgment of the unknown, fear of change instead of centuries of shared cultural wisdom. There is no humility, no uncertainty, in such a man.
A man of certainty, one whose theory must be correct for him to retain his place in the world, will kill as many as it takes to be right. The truth be damned.