Tonton Streaming Wallander (2008) S1E3 - One Step Behind

Three youngsters are shot dead whilst having a woodland picnic in fancy dress. The corpses are found, and a friend of the victims is also slain, having told Wallander that his workmate was really gay, and that he is a transvestite, and a wholly murderous one, who adds to his toll of dead by killing a further trio of innocents.