One day a person is really angry at somone, the next day they feel bad for them, can this be a sign of an underlying illness? If yes which one?
Well, it could be, certainly, although it would usually have to be in combination with other symptoms and criteria to really be cause for alarm. There are a number of emotional and brain disorders and illnesses that cause anti-social behavior or mood swings. The most obvious is bi-polar disorder, where we find large swings in moods and emotions, often within a short period of time.
Other important factors include the age of the person we're talking about. If they are over 55, then Alzheimer's becomes a possibility, as one of the symptoms is extreme and often irrational emotional irritability.
We have to be careful about reading too much into this one symptom. Maybe they're just having a bad day, or don't like this one person, or are irritated by something they said or did. It would have to be in combination with other symptoms.
The problem with anger bouts is that they are many times misdiagnosed. The first inclination is opting for bipolar or personality disorders of the kind, but anger has also been associated with:
Temporal Lobe Epilepsy (epilepsy with no seizures)
Borderline Personality Disorder
Beriberi (a condition caused by nutritional deficiencies)
ADHD not diagnosed (it can really aggravate adults especially who have no idea of what is causing their forgetfulness, or their inability to focus)
Like the previous poster said, check the age group of the person because Beriberi is usually associated with teenagers, whereas adult ADHD is seen more in adults over 30, and Alzheimers is a mature adult condition.
Check the link provided for more information. Hope it helps!