In what way can Juliet's deceitfulness be justified? How can her deceitfulness be viewed as negative? Provide textual evidence for support.

I think it's possible to justify Juliet's deceitfulness because she has relatively little control over her own life.  Her parents make most of her decisions for her, including who she should like or, later, marry.  When Juliet refuses their choice, her mother washes her hands of her, saying, "Talk not to me, for I'll not speak a word./ Do as thou wilt, for I have done with thee" (3.5.214-215), and her father calls her a number of terrible names and tell her that if she disobeys him, she can "hang, beg, starve, die in the streets" (3.5.204).  They are both incredibly harsh and seem to expect obedience rather than independent thought.  They would obviously frown on Juliet's choice, and so she feels that she must deceive them in order to be happy.

Further, her parents are insulting to her; do they even deserve her honesty?  Her mother tells her that she's crying too much over Tybalt's death, saying, "much of grief shows still some want of wit" (3.5.76).  It's true the Juliet is really crying over Romeo's banishment as well, but her mother doesn't know that and tells her that she looks like an idiot for crying so much.  Pretty insensitive: these are not parents that a kid can just open up to.

On the other hand, it is Juliet's deceitfulness that sets into motion the events that lead to her own and Romeo's suicides.  Friar Lawrence tries to speak with her about the dangers of drinking the vial he prepares for her, but she cries, "Give me, give me!  O, tell me not of fear!" (4.1.123).  She is quite willing to deceive her parents, the whole community in fact, and this deception fails when the Friar's letter doesn't make it to Romeo.