At the end of this week’s parshah, the Torah commands us to wage an eternal war against Amalek. The Mechilta states that Hashem swore on His throne that any non-Jew may convert and join the Jewish people, save for an Amalekite. Proof of this, states the Mechilta, is the story of David killing the Amalekite who informed him of Shaul’s death. The Amalekite told him he was a convert, but David remembered the halacha that we do not accept converts from Amalek and therefore killed him.
The simple understanding of this Mechilta is that David killed the boy because he admitted to being an Amalekite. Since an Amalekite may not convert, this boy was subject to the law that we must annihilate the nation of Amalek. However, the pesukim in Sefer Shmuel (II Shmuel, 1:16) make clear that David killed him for having killed Shaul, not because he was an Amalekite. How, then, do we make sense of this Mechilta?
Rav Moshe Shmuel Shapiro, zt”l, answers that there is no contradiction: David killed the boy for both reasons. Sefer Shmuel relates that Shaul was waging war and realized he was going to be captured. Rather than submit to this fate and suffer torture and humiliation, Shaul decided to fall on his sword. He did so but somehow remained alive. He then saw an Amalekite boy standing nearby and asked him to finish the job. Normally, this boy would not be liable for murder since Shaul was already a treifa (i.e., he was soon going to die regardless). Why, then, did David kill him? Because the law that a person is not liable for killing a treifa only applies to Jews, not to non-Jews (Rambam, Hilchos Melachim 9:4). And since an Amalekite may not convert, this boy was still a non-Jew. Thus, David killed him for two reasons – because he killed Shaul and because of the law that an Amalekite may not convert to Judaism.
Rav Moshe Shmuel offers another solution. Normally, a person may not be killed or lashed based on his own admission regarding non-monetary matters (Rambam, Hilchos Sanhedrin 18:6). This law, however, perhaps only applies to Jews. Non-Jews may perhaps be punished based on their own admission. Thus, again, David perhaps killed the Amalekite boy for two reasons: 1) Because he killed Shaul and 2) Because an Amalekite may not convert to Judaism and therefore this boy was still a non-Jew and could be punished based on a confession. (David only knew this boy killed Shaul because the boy told him so.)
The truth is it is not clear if a non-Jew is punished based on a confession. The Brisker Rav maintains that he is not. That’s why, he explains, Adam Harishon did not kill Kayin for killing Hevel.
The Rambam, however, apparently maintains that a non-Jew is not punished based on a confession. David killing the Amalekite boy was an exception to the rule. It was a hora’as sha’ah, a special circumstance, he writes.