Betis did take the lead after 18 minutes, however, when Paulao pounced on a loose ball inside the area before finishing from close range. The striker came close to adding his second of the game five minutes later, but he headed wide.
The away side's best chance of the half fell to Antoine Griezmann just before the 30-minute mark, but his effort was blocked en route to goal. Sociedad continued to threaten in the latter stages of the half – with Carlos Vela a threat – but they entered the break behind.
Betis came close to taking a 2-0 lead moments into the second period, but Castro stuck the bar with a fine effort from outside the box.
However, the away side remained a threat and Vela passed up a fine opportunity to level after 52 minutes when the former Arsenal man had been played in by Alberto De la Bella.
Both sides had chances in the latter stages, but it was Betis who made sure of the points when Castro headed home from close range. However, the victory was marred somewhat with a red card being handed to Mario in the final moments.