Purim

In Esther 2,22, why did Mordechai tell Esther to tell Achashverosh that Bigsan and Seresh were plotting to kill him? Why didn't he just let them kill Achashverosh? Avodah Zara 26 a-b says that you shouldn't save goyim's lives, especially Achashverosh's life considering (Rashi Megilla 14a) Achashverosh wanted to do destroy the Jews on his own, even without Haman's encouragement. Remember, this is before Haman became the second most powerful man in the world so chances are whoever would've taken over Achashverosh would've been much more friendly to the Jews. Maybe even Esther would've been the one who would take over, so why did they save Achashverosh from Bigsan and Seresh?