This comes from journalist Norman Cousins:
Nothing is easier than to denounce the evildoer; nothing is more difficult than to understand him. So wrote Dostoevski out of his preoccupation with the incestuous relationship between good and evil. William James pondered the same phenomenon and concluded that different persons have different thresholds of crossover from good to evil; no one can anticipate the circumstances under which his own crossover might occur.
Norman Cousins (1974). ‘In Defense of Father Citro’. In Saturday Review World 1:21 (29th June 1974), p. 4. Reprinted in Human Options (1981), p. 39.
The absense of quotation marks indicates, I think, that Cousins was summarizing Dostoevsky and James in this passage, not quoting them. But some readers misunderstood this, taking the first sentence as a quotation from Dostoevsky. The earlist instance I can find is Gerard Straub (1986), Salvation for Sale, p. 316, but it could have happened multiple times independently.
As for what Cousins meant by the difficulty of understanding the evildoer, it might help to look at the context of his editorial, which was the backlash to Father Joseph Citro’s eulogy for the terrorist Angela Atwood, a member of the Symbionese Liberation Army who had been killed in a battle with the police. (Citro had said that Atwood, like Christ, had died for her beliefs.) The difficulty of understanding is not an intellectual difficulty, as suggested in the question, but an emotional and social one: that is, how dare anyone try to understand the evildoer? You may recall a similar reaction to the 9/11 attacks, whereby any attempt to understand the motivations of the terrorists was reacted to as if it were an attempt to sympathize with them.