This is why you take seriously a person's threats -- at a workplace or anywhere else -- to go on a killing spree. Because sometimes they're not kidding.
More accurately, they may be "joking" at the time, but the statement may reflect an underlying desire or intent. Particularly when, as here, the statement is repeatedly made.
Pearson gets fired for telling his coworkers that he might go postal. Immediately upon being told that he was fired, he indeed goes postal.
He's sentenced to death, by the way. A decision that's unanimously affirmed in an 109-page opinion.