Midfielder Joaquin Sanchez threaded the ball to Castro who curved a shot into the net in fourth minute for his eighth goal this season.
Levante missed two chances to equalize five minutes later when Antonio "Verza" Garcia hit the near post only for Deyverson to also kick the rebound into the woodwork while speeding toward an open goal.
Levante goalkeeper Ruben Martinez tipped Sanchez's lob over the crossbar in the 34thand Deyverson hit the bar in the 55th.
Martinez made a full-stretch, athletic save in the 70th and Betis defender Heiko Westermann was sent off nine minutes later for a second yellow card.
Betis rose provisionally to ninth place.