Kurn didn't give his consent WHATSOEVER to be mindwiped, brainwashed and surgically altered to completely gain a new identity.
On the other hand, many posters here have already expressed the opinion that mindwiping was "as bad as" murdering Kurn. Kurn gave his consent to one kind of murder because he wished to die
, so it makes sense to assume the consent would extend to all types of murder, including mindwiping.
When the first murder was interrupted, those responsible did not yet know of the consent. Sisko was personally against the murder even after being educated about the context and the consent; apparently, Bashir was not. Which is fine and well, because neither Bashir nor anybody else we've seen in the 24th century has actually expressed anti-euthanasia sentiments. (Lwaxana Troi didn't like "forced euthanasia" in "Half a Life", but that's hardly relevant.)
It would be a bit much to interpret "Sons of Mogh" as taking a stance on euthanasia anyway. Klingons are always killing each other for weird reasons; this need not involve ethics or morals of any sort, nor should our heroes or the audience be interested in taking stances on the absurdities. That part of the story is just a vehicle for Worf to get into conflict with the Klingon Empire in yet another entertaining manner.