It is impossible to picture to oneself the shame and moral degradation to which the jealous man can descend without a qualm of conscience.
 Fyodor Dostoyevsky, The Brothers Karamazov (1880). copy citation

Context

“But Othello did not begin hiding, spying, peeping. He was trustful, on the contrary. He had to be led up, pushed on, excited with great difficulty before he could entertain the idea of deceit. The truly jealous man is not like that. It is impossible to picture to oneself the shame and moral degradation to which the jealous man can descend without a qualm of conscience. And yet it's not as though the jealous were all vulgar and base souls. On the contrary, a man of lofty feelings, whose love is pure and full of self-sacrifice, may yet hide under tables, bribe the vilest people, and be familiar with the lowest ignominy of spying and eavesdropping.” source