In fact, many American Communists were simply people driven by a deep concern for human justice. If, for example, you went into the south before the civil rights movement and found a white working on the issue, it would not be surprising to discover that the activist was a member of the Communist Party, about the only one that cared at the time. Even in the 1960s, it was not unusual to run into former Communists providing important leadership, using their years of activist experience.
Were these evil people? Far from it. They were among the decent people in politics. Many were in the arts strong, sensitive and deeply idealistic. Others were in the labor movement helping unionists become so successful that more than a few would end up voting Republican. You can't tell the story of American social democracy without the story of American communism.
Where the trouble began was not with domestic politics but with the foreign. Precisely because they were so idealistic, many had a hard time melding ideology with what was actually going on. Even today, there are echoes of this in left debate: a conflict between intellectually-based and reality-based discussions of politics - issues of faith versus those of fact.
Nowhere was this more striking than with American Communists who defended the Stalin regime. A handful engaged in espionage they justified by their beliefs, but most simply tolerated, excused, or explained away the Soviet beast. The fact that Adolph Hitler was there as a convenient negative comparison didn't hurt.
Lest one become too sanctimonious about this, however, it is useful to compare the naivete of American Communists with that of the American establishment which has supported an extraordinary line of dictators and other monsters and helped create more such as Saddam Hussein and Osama bin Ladin.
As one who, at the age of 23 because of his parents' not atypical associations in the Washington of the time, found himself a victim of the Communist hysteria foisted by the FBI and others, I have long understood how distorted the story has been.