Just hold people responsible for their direct actions and keep what they did in perspective. The people that Kazan informed on weren't just political dissidents, but fans of Joe Stalin's murderous regime-and they "suffered" far less than any kulak or refusnik.
Robeson is often lionized for having been "persecuted" without any acknowledgement of the nastiness of his politics, he's a scumbag regardless of how great an althlete or entertainer he might have been.