“Boardwalk Empire” offers no shortage of compelling characters, but Irish lad Owen Slater was a standout. He protected Nucky from enemies while simultaneously wooing Nucky’s wife, the lovely Kelly Macdonald. Since he was so involved in both their lives, we figured he would be around a while. But like Jimmy Darmody’s fate, we figured wrong. He ended up in a box — a la Gwyneth Paltrow’s head in “Seven” — creating a scene that was shocking in its audacity.