1)
What is the meaning of "Hacharesh"?
1.
Radak: This is Hifil, Po'el Omed (the action applies to the subject himself), and similarly "Hacharishu Mimeni" (Iyov 13:13). In most cases, the Hifil of this verb is Po'el Omed.
2)
Why did they think that Shmuel would not pray for them?
1.
Malbim: They saw that he trusted in Hashem to overpower them. He was confident that his prayer was already accepted. They said, pray that Hashem will save us by Himself. Since we are afraid, we cannot defeat them in war!
