An actor wants to leave the series, so the writers get together to plan his exit. "Let's do something original, let's kill him off." You have got to be kidding me. That has been done a million times. The FBI transfers people all the time. Why not transfer him or medically retire him due to the shooting or have him retire due to the shock of the shooting. In any of the scenarios he could come back as a guest star. However, killing him off probably seems easier because they don't have to think too hard.