The man with the gun was just a decoy. A lot of noise amounting to nothing. While everyone was dealing with him the witches slipped into the museum. Incantations and potions were deployed undetected.
At first nobody noticed the change, but slowly the customers stopped coming. Some complained of the museum’s new “creepy” feeling. Then the donors stopped donating. The joy had gone out of the galas. The art had lost it’s appeal.
The board members would for the rest of their lives regret rejecting that one witch’s painting. Some cried when the bulldozers came. The witches laughed and sang.