1)
What is the meaning of "v'Hemah l'Sho'ah Yevakshu Nafshi"?
1.
Rashi: My enemies ambush me on a dark day, lest I notice them.
2.
Radak: My pursuers sought to kill me and make me desolate.
3.
Malbim: My dear Nefesh, whose primary goal is clinging to Hashem, they seek to make it desolate and dark - to uproot it from Artzos ha'Chayim.
2)
Why did he say "Yavo'u b'Tachtiyos ha'Aretz"?
1.
Rashi: In a low place in the depth, they come and fall.
2.
Radak: This is a Tefilah; they should come to the grave, which is under the ground, they who want to make my Nefesh descend to the grave.
3.
Malbim: They go under the ground, in caves and tunnels in the Midbar, in search of David.