The Dalish use "Harellan" to mean "traitor to one's kin," but the word does not appear in any elven text before the Towers Age. The ancient root-word is related to "harillen," or opposition, and "hellathen," or noble struggle. The Dalish call Fen'Harel a god of deception, but I posit a far more accurate translation would be "god of rebellion."
What he rebelled against is a story lost to time. In Dalish legends, Fen'Harel seals away the other deities out of love of trickery. If we understood more ancient elven, we might find earlier versions of the Dread Wolf's story give him a more nuanced motivation beyond spite.