Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to a Human you control may become lethal if General Kudro leaves the battlefield during that turn.

Only creatures are Humans. Planeswalkers such as Lukka and Narset who represent human characters won't cause General Kudro's middle ability to trigger.

General Kudro can be sacrificed to pay the cost of its own last ability. How noble.