Even if Real Madrid does not use their key players to play against betis, I don't think betis will defeat Real Madrid. Real Madrid performances are still alright without their best players. If betis tend to be tough in the game, it will end up with a draw . The game will likely be like the previous game Real Madrid played against cadiz, it was a game that ended with a draw but Real Madrid dominated and control the game more.
Betis have a different level of play with Cadiz. if Madrid is not serious about the match against Betis, this should be an opportunity for Betis to get points.
although Madrid will focus on the Champions League final. very not good if they have to end the season with a result that is not optimal. moreover, they are already partying for the La Liga Trophy this season. end the season with a win I think Madrid will do.
Even if Real Madrid main focus is on the champions league, am not sure they still will not take the match against betis serious. Real Madrid is a team that always want to be called that winning team. After they play draw with cadiz it will look some if they should give betis the chance to win, so they have no choice to try to be serious.