1)
Why does it say "Choshech va'Chalaklakos"?
1.
Rashi: Both [darkness and slipperiness] come together, so their feet will slip, and due to the darkness, they cannot guard from it - "Yihyeh Darkam Lahem ka'Chalaklakos ba'Afelah" (Yirmeyah 23:12).
2.
Radak: Each of them by itself is a problem. In darkness, one trips of rocks and obstacles. One who walks on a slippery path, he slips and falls. Perhaps one who goes gently can avoid both of them. However, "u'Mal'ach Hashem Rodfam", so they must run, and will surely fall.
3.
Malbim: Their path should be dark and slippery, so they will fall by themselves. Additionally, a Shali'ach of Hashgachah [should help make them fall] - "u'Mal'ach Hashem Rodfam."
