Let Rahu be dharmic or adharmic. The thing is that we are all trying to explain dharma and adharma according to one's own mindset, ideas and mythologies, theories, learned histories, but the truth maybe far from that because what is dharma and what is adharma counts only as per his texts and not of human made or human minds.
In some rare cases, one cannot say this is dharma and this is adharma etc. In some cases, there are stages where one cannot teach dharma as well as in the context of the game, it maybe vital.
For e.g. Lord Krishna told Arjuna to bow at Karna when Karna was without any Shastras or Asthras in the ground. As per the laws of war, some might say what Lord Krishna had done is adharma as one is not supposed to bow at the opponent when he is Nirayudha, but only he knows why and what?
So, describing everything according to our logics might not be correct, but yes Rahu definitely has the conventionality of doing things on his own, isnt it?...
All these theories are confined to boundaries where the logics will also fail beyond it.