At this point, the Romans really went crazy. Yes, they went nuts, bananas, watermelons, whatever you like to call that condition. They started looking for a culprit, a scapegoat, someone to blame. Now, you remember that Emperor Honorius had accused his general Flavius Stilicho of treason; that is, of being colluded with the Barbarians. That was already an effect of rampaging paranoia. But, in the besieged Rome, paranoia went up of a few notches. Someone noticed that Stilicho's widow, Serena, was in Rome. If her husband had been a traitor, well, she had to be a traitoress. Serena was the cousin of Emperor Honorius, a noblewoman of high rank. But when paranoia becomes the rule, it generates pure evil. Serena was accused of treason, sentenced to death by the Senate, and executed by strangling.
It is at this point that we have the first appearance of Galla Placidia in history as an adult, she was around 20 years old at that time. We are told by the chronicler Zosimus that the execution of Serena was done "with Galla Placidia's consent."