Why are sick people so mean?

Part of this has to do with the prefrontal cortex of the brain. A healthy person doesn't have much of a load on this part of the brain, since they don't have to fight a lot of pain and stress. This part of the brain is what filters inappropriate behavior. Nearly anyone can be nasty, but the front of the brain is trained to where you can hold this back. Now, if you are actively trying to ignore pain, this part of the brain is busy. So a person's responses and behavior will be more raw, since they will come more from the limbic system and be unfiltered.