From TF2 Wiki
A Destruction is a type of kill earned after destroying a building, either using a weapon or an electro sapper. A destruction is worth 1 point and can not be assisted unless the building has been sapped before by a Spy in which case the Spy gets the assist. You cannot get a destruction from destroying your own buildings. Destroying a building that is not yet completed still yields a destruction.
When a building is destroyed, the Engineer responds with one of three responses, alerting his nearby teammates.