Having recently read C. S. Lewis' The Pilgrim's Regress, I found a couple of quotations about extremism that I'm saving here.
I do not admire the excess of one virtue unless I am shown at the same time the excess of the opposite virtue. A man does not prove his greatness by standing at an extremity, but by touching both extremities at once and filling all that lies between them. — Blaise Pascal
Opposite evils, far from balancing, aggravate each other...widespread drunkenness is the father of Prohibition and Prohibition of widespread drunkenness. — C. S. Lewis