[SCRIPT] destroying key after use?

[Xanthos]

The Engine Scripts Page is there to help you! It is RemoveItem(""); where is the name of the key, in this case key_study_1. So it's RemoveItem("key_study_1");

so i put it underneath void UnlockDoor?

Yes
Put it under the SetDoorLocked

It's also good to add.
GiveSanityBoostSmall();
PlaySoundAtEntity("", "unlock_door", "player", 0, false);