1)
Why did he say "Chalilah Li? mi'Shelo'ach Yadi?" - he already said like this (verse 9)!
1.
Malbim: Do not say that there is no punishment for killing him 1 , for he is pursuing me. There is another way to save myself, through taking his spear and flask!
1
Above (verse 9), Malbim said 'even though he is a Rodef and one may kill him, one who kills Mashi'ach Hashem will not be cleansed.' That was before David saw another way to save himself. Then it is not murder, just there is a punishment for killing Mashi'ach Hashem. Now that there is another salvation, it is murder to kill him, even if he were not Mashi'ach Hashem! (PF)
2)
What is "Tzapachas"?
1.
Rashi: It is a flask.
