THOUSAND OAKS : Cause of Thornton’s School Ills Disputed

Under a tough cross-examination, a psychologist maintained Tuesday that a learning disorder was the most likely reason convicted murderer Mark Scott Thornton picked fights with classmates and lied to his teachers as early as elementary school.

A prosecutor, however, suggested that the behavior indicates that Thornton has been mean and dishonest from an early age.

The exchange came before a Superior Court jury that is deciding whether Thornton should be put to death for the September, 1993, shooting death of Westlake nurse Kellie O’Sullivan.

The jury, which convicted Thornton of the murder in December, also has the option of sentencing the defendant to life in prison without parole.

Prosecutors say O’Sullivan was kidnaped in Thousand Oaks after being ordered to her knees in a remote location of the Santa Monica Mountains. The defense contends Thornton suffered brain damage at birth and, as a result, is an immature adult who cannot control his impulses. They attribute his actions to a short attention span, among other factors.

But Deputy Dist. Atty. Peter D. Kossoris read from elementary-school records that showed Thornton would often pick fights with classmates and try to cover up his behavior by refusing to take responsibility for the misdeeds.

Kossoris pressed psychologist Brian Jacks, a defense expert, on the cause of the defendant’s behavior--suggesting it resulted from spitefulness, deceitfulness and selfishness.

But Jacks, repeating his testimony from a day earlier, said Thornton suffered from an attention-deficit hyperactivity disorder that could also cause such behavior.

The hearing, in its third week, will continue today.