How did Mordechai know?
Rashi: He was told in a dream that above they agreed [to Haman's decree] because [Yisrael] bowed to the image in the days of Nebuchadnetzar, and benefited from Achashverosh's party 1 .
Malbim: Via Hashgachas Hashem. Hashem 'laughs' at all Haman's efforts to keep the matter a secret. Refer to 4:1:2:2.
Megilah 12a: R. Shimon says, the decree was because they bowed to the image. His Talmidim said, it was due to benefit from the party.
What does "Kol" include?
Vilna Gaon: That [Haman wrote the decree] because Mordechai did not bow.
Malbim: He knew how Haman tricked the king, and all his schemes and strategies.
Why did he tear his clothes, put on sackcloth and ashes, and scream?
Malbim: This was for Teshuvah and Tefilah, to arouse desire from the One in whose hand are kings' hearts.
What is the significance of "Ze'akah Gedolah u'Marah"?
Torah Temimah citing Esther Rabah: [Via taking the Brachos,] Yakov caused Esav to scream 1 "Ze'akah Gedolah u'Marah" (Bereishis 27:34). He was paid up in Shushan (Haman is from Amalek, Esav's grandson). This teaches that Hashem does not overlook.
Vilna Gaon: It was Gedolah due to the great affliction. It was Marah Mordechai knew that it came due to himself.
Yakov was obligated to take them. His mother commanded him, based on Ru'ach ha'Kodesh! Perhaps Yakov was liable for not empathizing with Esav's pain. Or, Yakov did not merit to receive the Brachos without causing pain to Esav; this pain was returned to his descendants. (PF, heard from R. C. T. Fogel and Rav Weisman).
What did Mordechai scream?
Torah Temimah citing Megilah 15a #1: Haman exceeded Achashverosh (regarding his evil plans for Yisrael).
Torah Temimah citing Megilah 15a #2: The higher king overpowered the lower king (this is a euphemism for the opposite).
Malbim: He publicized the decree, so also Goyim and the king would find out, and advocates will rise to stop the decree.
