It says in the Mishna, in Chapter of the Fathers as translated by Chabad
http://www.chabad.org/library/article_cdo/aid/682520/jewish/English-Text.htm18. One who causes the community to be meritorious, no sin will come by his hand. One who causes the community to sin, is not given the opportunity to repent. Moses was meritorious and caused the community to be meritorious, so the community's merit is attributed to him; as is stated, "He did G‑d's righteousness, and His laws with Israel" (Deuteronomy 33:21). Jeroboam the son of Nebat sinned and caused the community to sin, so the community's sin is attributed to him; as is stated, "For the sins of Jeroboam, which he sinned and caused Israel to sin" (I Kings 15:30).
So even if it were true that the Castros have Jewish blood in them from their mother's side (the theory is that they descend from Jews who were forced to adopt Christianity in Spain see
http://www.jewishcuba.org/marranos.html, I still don't have sympathy for them given the tremendous Jeroboam style spiritual damage done to the community and for helping along the war against G-d, as well as being thieves and killers.
True, they could have been even worse to the community, but they were still were bad and will be held to account for the evil deeds they did in practice, when G-d judges them.
I hope bli neder, to present in the future what is the status according to Halacha of the Marranos of Spain and their descendants elsewhere in the world.