IMHO, Bashir acted within the bounds of the Hippocratic Oath. Assuming he took the phrase "do no harm" to mean "preserve the patient's life", then what he did certainly qualified as that, because if he didn't wipe Kurn's memory, Kurn would have died. So the act of wiping said memory preserved Kurn's life, therefore, no harm was done (according to human medical ethics).
As for Worf killing Kurn: I maintain that they should have gone to Bajor, then they could have legally carried out the ritual. On DS9, it wouldn't work, because while the station is owned by the Bajorans, it's run by the Federation, so it's considered Federation territory. Bajor, at that time, was not a Federation member, and I doubt they would have interfered (heck, for all we know, Bajoran law might contain a similar honor code!), so if Worf had killed Kurn on Bajor, the Federation could not do anything about it.
Actually, what would really be interesting is if we later found out that the wiping of Kurn's memory was actually done with Kurn's own consent. Maybe Kurn chickened out and didn't want to die, so at the last minute he allowed Bashir to do this procedure, knowing that he would never remember it (and thus not consider his life to be without honor).