How would you rate Carlsen's actions, relative to your understanding of chess ethics?
The ethical side really boils down to the question, "who is hurt by these actions".
If it is a Swiss tournament and he just silently withdraws then none of the other players are affected. The player tells the arbiter he is withdrawing, the arbiter marks him as unavailable for pairing in the pairing program and the tournament continues. If it counts as bad publicity (unlikely) then the sponsors of the tournament may be damaged but normally all extra publicity counts as good publicity. So, for instance, rage-quitting Titled Tuesday punishes nobody.
But this was a round robin tournament. An early withdrawal meant players who were paired to play him in later rounds had the chance to play him removed and ended up playing one game fewer. Any player doing this in a norm tournament (a round robin tournament set up to allow players the chance to achieve title norms) would basically ruin their own reputation. They would never be invited to such a tournament again because they likely ruined the norm chances of the norm seekers who paid good money for the chance to achieve a title norm.
Although none of the players at this level are norm seekers his actions still caused disruption and harm to the players and the tournament.
However in this case there is an additional ethically bad element. His actions were a silent (maybe not so silent) accusation of cheating. The method was cowardly. Instead of coming straight out and presenting evidence that his opponent had cheated he implied that using innuendo.
Normally in a tournament if a player comes to the arbiter or organizer and accuses another player of cheating that accusation is kept secret unless or until definitive proof is obtained that that the player was cheating. To publish unsubstantiated accusations like that amounts to defamation and in some countries could result in legal action. It looks like Carlsen came as close to that line as possible without actually crossing it.