De Soto, 21, was arraigned Wednesday on three counts of aggravated murder and 11 counts of attempted aggravated murder, the Guam Pacific Daily News said.
The Daily News said defendants in Guam are allowed to plead not guilty and not guilty by reason of insanity at the same time.
De Soto allegedly hit people with a car on Feb. 12, then crashed the car into a store and stabbed bystanders with a knife, the Daily News reported.
Just before his arraignment Wednesday at the Superior Court of Guam, De Soto allegedly attempted to escape from marshals, the Daily News said.
A court spokesman said De Soto made some sudden movements when a marshal removed his handcuffs for court, but additional marshals were able to control him.
If De Soto is convicted of any of the aggravated murder charges, he will face a mandatory sentence of life in prison without parole, the Daily News said.