Question: Why doesn't Kay kill Anderson after he grabs her and takes the elevator up to Ma-Ma? Or at least, why doesn't Ma-Ma kill her when Kay comes in with her? They were intent on killing the Judges and didn't care about the consequences. So what was Ma-Ma hoping to accomplish having her as a hostage that she couldn't try to accomplish without one? At this point, she already had the 4 crooked Judges on their way.
Question: Why would Jesse forgive Beca just because she sang a song he likes?
Answer: It wasn't just that she sang a song he likes, she sang arguably the most well-known song from his favorite film in front of a huge crowd of people. It was a public, heartfelt apology rather than the abrupt, private apology she gave earlier.
Answer: Kay wanted to punish her for what she did to him earlier. Ma-Ma wanted leverage against Dredd should he get past her Judges.
MasterOfAll