He ran the booth that guessed peoples weight. Under that pretense to see if he's right, he looks up their addresses to rob them. One died of a heart attack as a result of his break-in and he killed another on a different break-in.
Rooney was a carnival worker who was Dexter's first victim shown in Season Three. He appeared briefly in flashbacks while Dexter recounted in his usual ambiguous way what he did over the summer to his dentist. Dexter had injected the man, who looked to be in his early 50s, with a tranquilizer, taken a blood sample from his cheek, and killed him with a meat cleaver.