There probably was a good reason. Then again judging how Damien reacted when the source of his power was stolen I don't think he and sanity have been in the same room in a super long time. Based on feats he should kill Ras.
__________________ "I know it's gonna work because it's impossible"-George Lucas
Well, Thea showed resistance to his abilities after being revived by the Pit. Maybe over a century of constantly being healed and kept young has given Ra's a similar kind of resistance? I really have no idea.
Without his powers, Ra's would absolutely murderstomp Damien though. Darhk got his ass kicked by a pair of prison thugs.
__________________ Never take life seriously. Nobody gets out alive anyway.