Reading Transmetropolitan: Gouge Away, I found this quotation:
The most dangerous man, to any government, is the man who is able to think things out for himself, without regard to the prevailing superstitions and taboos. Almost inevitably he comes to the conclusion that the government he lives under is dishonest, insane and intolerable, and so, if he is romantic, he tries to change it. And even if he is not romantic personally he is very apt to spread discontent among those who are.
(Smart Set Magazine, December 1919)
Lots of people seem to have gathered Mencken quotes (1400+ in Google). Here are some particularly worthwhile collections: