He is an artist who married an older, successful woman by the name of Vanessa who later committed suicide.
Alex's wife proved to be a powerful prosecutor, putting away countless criminals. However, the stress forced her into therapy with Dr. Emmett Meridian. Little did Alex know that the doctor was manipulating her through drug prescriptions and thoughts of suicide. This led her to kill herself without Meridian physically committing the act himself.
It came to Dexter Morgan's attention that Vanessa was the third powerful woman to commit suicide in this way, with two having shot themselves the year before. Since most women don't commit suicide by using firearms, Dexter began to investigate the cases. He made the connection by discovering Meridian's involvement. He avenged the three victims when he killed Dr. Meridian. At the same time, he satisfied his own dark urges.