Currently the raid icon appears for the entirety of the raid duration, regardless of whether you kill all the creatures or not. So think of the raid icon as more of a timer for how long the event can stay active, rather than it being an indication of whether the creatures are still alive or not.
So if you see a raid icon all it means is that the raid was activated and it may or may not have been killed by other players already. The icon will stay there for the set amount of time the raid is supposed to last, and there is no player input that will change it.