I am playing the third encounter set of hobbit over hill and under hill and I have earned sting. when I get sting and place it on Bilbo I gain an ability to when I defend I draw a card from the encounter deck and place damage on the attacker equal to the threat number. when I am attacked by golium by not getting a riddle right I use Bilbo to defend. if he has sting he places damage on golium. what happens after I place enough damge on golium to kill him while I am still playing the 2nd quest card?

Dont have the cards in front of me, but if i remember correctly you are not allowed toplay cards on bilbo during that stage of the quest, so you can't destroy gollumn then.

Tracker's right, Bilbo is in his own separate area and you are not allowed to play any cards on him, including Sting.