well... kingmking is not easy to judge. IMO player should always play to his advantage.
Ensuring that winner has lower score means player has better tiebreaker score (higher proportion of winning points) - so if his behavior could give him better proportional result, it seems fully justified.
Likewise - within round, total scores count. If he can maneuver player order to have a shot at better place (whether to hold current winner to have hope of surpassing him, or coversely prop up winner to ensure better chance of not being last) - it IS kingmaking technically, but also seems fully justified to me (it's not giving advantage to other player out of spite, but rather the best possible play as defined by tournament conditions).
Last but not least, "being clearly third" and "trying to achieve as much as possible" do not exclude each other. Player may prefer to be 3rd with 200 points out of 300 than 3rd with 200 points out of 400, even outside tournament (and effect would be the same as with point 1 above).
Last but not least - "playing AI" is debatable... but also, it would be a poor play not to use the opportunity to achieve more (however "more" is defined, as above). If AI resigned, it couldn't be maneuvered to any player's (dis)advantage.
