Why does it say "Al Mishkavo"?
Radak: The bed is the place of thought, as long as he is awake, for he is not engaged in any matter. "Yisyatzev Al Derech Lo Tov" is during the day.
Malbim: Based on man's nature, he should understand the Emes and improve his ways, Due to sinful thoughts on his bed, and not going on the good path, a bad nature sprouts in his heart.
Why does it say "Yisyatzev Al Derech Lo Tov", and not 'Ra'?
Rashi: (His path is not only bad - it is of death!) Hashem put in front of him the good path, and the path of death, and he chose the path that is not good.
Radak #1: [The verse prefers to avoid derogatory words,] just like it says "Asher Einenah Tehorah" (Bereishis 7:8, in place of 'Temei'ah', even though the latter is more concise).
Radak #2: This was to avoid saying 'Ra' twice consecutively (after this, it says "Ra Lo Yim'as").
Radak #3: There is no good aspect of his path; it is totally evil.
Malbim: Even though he does not overtly do evil, since he is not going on the good path, evil sprouts in his heart.
