LONDON — A former trader at UBS accused of causing a multibillion-dollar loss was ‘‘playing God’’ with the Swiss bank’s money, prosecutors told a jury here Thursday.

During closing arguments in the eight-week court case, Sasha Wass, the lead prosecutor, said that Kweku M. Adoboli, a former UBS trader in London, had ‘‘arrogantly’’ sidestepped the firm’s rules and carried out risky trading activity from 2008 to 2011.