Full Idea
Reasonable and vicious are quite consistent with each other, in fact, only through their union are great and far-reaching crimes possible.
Gist of Idea
Reason can be vicious, and great crimes have to be rational
Source
Arthur Schopenhauer (On the Basis of Morality [1841], p.83), quoted by Christopher Janaway - Schopenhauer 7 'Against'
Book Reference
Janaway,Christopher: 'Schopenhauer' [OUP 2002], p.90
A Reaction
This is opposed to Kant, who always looks wildly optimistic in his hope that high rationality entails a morally good will. Good people seem to have a fairly irrational empathy with their fellow citizens.