That police officer lost his temper real quick, and for what? A comment someone made? Just goes to show you how "professional" the police are. If something as little as a "God bless you" can pull his strings--imagine what it takes for him to pull out his service weapon and kill someone?
Every one has their share of a "bad day" and it is understandable that a police officer would be no different. With that being said, I think it is very important for the police to be able to keep their cool as they are very visible to the public and people generally will notice what a police officer is doing long before they notice what a "normal" person is doing.
This plus the fact that a police officer is likely going to encounter a high stress, high stakes situation when many people's lives are at stake, and if they cannot keep their cool then they may not be able to make a good decision