He was upset that he would not be the only child anymore. When his baby sister was born Eva held her in her hands to get her to stop crying. Once Kevin realized she stopped crying he threw water in her face to make her cry again. Cognitive theorist would say Kevin held an attitude that diminished the importance of other people's needs. Eva seemed to always have flash backs on the major life stressors she had in life due to the medications she was intaking. In the movie it did not show what exact type of meds she was on or what the reason it was for but it did have an impact on the child's brain functioning. Biological theorist would say that there are hormonal and brain abnormalities. In other words these abnormal brain chemistries run in the family. Kevin never talked about any friends, he never hung out with anyone or developed a relationship with anyone outside of his father. In the eyes of a sociocultural theorist they would say Kevin may have been bullied in school but never brought it up so now he does not want to be associated with anyone.
In conclusion the message this movie sent is a lesson learned. Plenty of kids act up for attention was it because Eva did not show Kevin the love and affection he needed to become a normal child