On Purim, we celebrate our salvation from Haman’s efforts to annihilate the Jewish nation. Because he was descended from Amalek, on the Shabbos before Purim we publicly read the mitzvah to remember what Amalek did to our ancestors and to completely obliterate them. The Haftorah read on this Shabbos records Shmuel’s instructions to Shaul to kill all the Amalekites and their animals.
Before Shaul attacked Amalek, he first told the Kenites to move away. Rashi explains that the Kenites were descendants of Yisro. Because they lived near Amalek, they could accidentally be killed in the upcoming battle, so Shaul sent a message telling them to relocate. He added that it was appropriate to give them this warning because Yisro helped us by welcoming Moshe into his home in Midian and later advised him regarding the establishment of a system of judges.
Rav Dovid Cohen adds (Ohel Dovid Vol. 3) that Rashi writes (Shemos 18:1) that Yisro was motivated to join the Jews in the wilderness when he heard about the war against Amalek. Since the fight against Amalek was part of the inspiration for Yisro's conversion, Shaul felt it would be inappropriate for his descendants to be killed due to a battle against Amalek.
Rav Eliyohu Dessler notes (Michtav M’Eliyohu Vol. 4 pg. 2) that this is even more surprising when we consider that Yisro’s chesed took place more than 400 years before this incident. At this point, one could argue that the Kenites in the times of Shaul no longer had any connection to Yisro and did not deserve his merits, especially after they opted to move next to our archenemy, Amalek. This teaches us that we must never forget a favor that was done for us, no matter how much time has transpired.
The Mussar HaNevi'im suggests that Shaul’s compassion for Yisro's descendants also addresses another potential concern. A person who hears about this episode could view the Jewish people as exceedingly vengeful, as they endeavored to destroy an entire nation because of something that was done to their forefathers by Amalek's ancestors many generations earlier. To show that we are not just focused on avenging a wicked act that was done in the distant past, Shaul demonstrated that he was equally concerned with remembering the good that was done by Yisro and wanted to protect his progeny due to the chesed he did for us. (R’ Ozer Alport)
