Since sleeping and dreaming are surprisingly risky activities in Faerie Land (check out Morpheus for more), it follows that Night is also a shady—literally!—figure. In addition to helping out the evil Duessa, Night is indirectly responsible for causing a lot of trouble since she's the ancestor of several major villains: The Sans-brothers, Pyrocles and Cymochles, and even Duessa herself.
So yeah, you can blame Night for quite a bit in this text. Since light and truth and are often associated with light (as is Jesus) Night, the absence of light, is often associated with the opposite of those things: secrecy, immorality, and unreliability. Night is also associated with death, and sure enough, Night spends most of her brief appearance in the poem in hell, suggesting she's pretty familiar with the place.