Humble Defector's ability can be activated any time during your turn, including in response to a spell or ability.

If Humble Defector isn't on the battlefield as its ability resolves, but the target player is still a legal target, the ability will resolve. You'll draw two cards, even though the player won't gain control of Humble Defector.

If Humble Defector is controlled by a player other than its owner, and its controller leaves the game, the effect giving that player control of Humble Defector ends. Humble Defector will return to the control of the player still in the game who most recently controlled it.