1)
Why is Resha'im's path compared to darkness?
1.
Mesilas Yesharim (Perek 3): Darkness covers the eye, so he does not see what is in front of him at all. So physicality of this world, it is darkness for the eye of the Sechel. It does not let him see stumbling blocks in the ways of the world.
2.
Malbim: It seems that they go on a wide path; initially it is flat. Even so, it is Afelah, which is darker than Choshech. Even their wide Derech (general deeds) are full of darkness, and all the more so their particular deeds.
2)
Why do they not know on what they stumble?
1.
Rashi: A stumbling block comes suddenly. They did not know to beware of it.
