Trite, but true. When people are passionate and think they've got it figured out, they grasp on examples that justify and validate their beliefs. When things don't work out the way they expect, or when the data starts coming in indicating they chose poorly, they will bend and construe what they see in order to frame the phenomenon. The contortions they go through can be quite extraordinary, but we must beware about blaming it all on the ideology alone. It is strict adherence to the ideology, masquerading as principle, that is the monster. It happened to leftists with Stalin, and later with Castro. Once the truth of the personality or regime starts to come out, they refuse to see it, make excuses for it, and otherwise make fools of themselves trying to save the phenomenon.

The enemy is loyalty to an ideology, ideology should be a preliminary filter to aid us in evaluating and interpreting new data, just like a working hypothesis or a preliminary theory is in science. It must never become a set of laws which govern the Universe and must be preserved at all costs.

There was no reason why a leftist couldn't despise Stalin, and see for himself what he was doing. George Orwell did and his Socialist bona fides were impeccable, so did Trotsky, for that matter! But a whole generation of his contemporaries disgraced themselves by swallowing the party line out of Moscow. The same can be said for those who still make excuses for Castro, and his dead saint, Guevara. I will graciously refrain from pointing out the analogous contradictions on the right, we all know them well enough.