In the Bible, Mordecai is the uncle of Queen Esther (a Jewish queen of the Persians). He is considered a Jewish hero because of his role in stopping a Jewish genocide instigated by Haman, a top Persian official. The event is celebrated every year at Purim, the Jewish equivalent of Mardi Gras.
Needless to say, I don't think the WWE Mordecai has anything to with the biblical Mordecai. I think they just chose the name because it
sounds all evil and threatening.