The former Olympique Lyon man sent a bullet header past keeper Toni Doblas in the sixth minute to give his side a lofty view for at least 24 hours.
Despite the bright side, Real quickly dropped into cruise control against a lethargic looking Betis side that never really threatened Fabio Capello's men.
Betis, who have now won only one of their opening four matches, finished with 10 men after midfielder Nano was shown a red card for elbowing Diarra in injury time.
The Madrid side's only concern was the knee injury suffered by Brazilian right back Cicinho after he was carried off on a stretcher after the first half hour.
"We think he has torn the anterior cruciate ligament, but we will have to carry out further tests tomorrow," Real Madrid doctor Juan Carlos Hernandez said.
The match was also marked by the appearance of David Beckham on the bench for his second successive match.
The former England captain's usual place on the right wing was taken by Spanish winger Jose Antonio Reyes who has impressed since joining from Arsenal in the close season.
It was the first time during his time with Real Madrid that the former Manchester United star has been on the bench for two consecutive matches.
Four teams will vie to displace Real from top spot on Sunday.
Champions Barcelona and Valencia, two of only three teams with a 100 percent record this season, meet at the Nou Camp in the game of the weekend.
Sevilla, the other team with a perfect start, travel to a rejuvenated Atletico Madrid.