1)
Why did David not keep people alive for slaves?
1.
Malbim: It is like verse 11 says, lest they tell Achish. 1
2.
If they were from the seven nations, like Malbim said, it is forbidden to keep them for slaves - "Lo Sechayeh Kol Neshamah" (Devarim 20:16)! (PF)
1
Malbim (verse 8) explained that Pelishtim would not object to killing these people! Even so, Achish would be upset that David lied to him, and would not trust in him, especially if he sensed that David did not kill Yisraelim at all. Why did Malbim not say that he killed them due to Lo Sechayeh?! (Refer to 27:9:1:2. - PF)
2)
Why did he come to Achish after killing them?
1.
Malbim: He would tell him.
