Malbim: They thought that Gedalyah commanded to kill them for going to Beis Hashem. They offered a bribe if he will spare them. Yishmael spared them to affirm this. People should think that Nebuchadnetzar appointed Gedalyah to kill mourners over the Churban, and they will rejoice that Yishmael killed him and endorse his killing Gedalyah, who killed people without proper reason.