why is shadow considered sinister, unpure, and deceptive?
It's only "considered to be" that way because that's how you see it. I'm certain that there are many people who do not see things that way. Example - in the movie "V for Vendetta" (great movie, BTW) the character "V" only operated in the night time, and most of that movie was filmed at night. Conflictingly, the character (who performed many acts of terrorism, violence, and murder) was ultimately using those acts as tools of justice (seen as good, or "daytime" to some). As a clarification of humans' unsubstantiated fear of night time and of darkness:
Night is often associated with danger and evil, because bandits and dangerous animals can be concealed by darkness. The belief in magic often includes the idea that magic and magicians are more powerful at night. Similarly, mythical and folkloric creatures as vampires, and werewolves are thought to be more active at night. Ghosts are believed to wander around almost exclusively during night-time. In almost all cultures, there exist stories and legends warning of the dangers of night-time.
Lewis Carroll wrote:
Still she haunts me, phantom-wise, Alice moving under skies; Never seen by waking eyes... Name origins