However, as Spencer points out, there is also evidence that Melissa is far more innocent than it seems. While we're not ready to give the elder Hastings girl a pass, here are a few reasons we're also not convinced she definitely is involved with "A."
Plausible explanations: In this sneak peek from Episode 22, Melissa provides Spencer with some very plausible explanations for her actions: She saw Garrett because he's one of the few people around who doesn't totally hate Ian (Ryan Merriman), she texted Ali to tell her to back off from Ian, nothing more, and we can extrapolate that she was at the house that night to yell at Ali about Ian.
Sure, these could be well-crafted lies, but it's equally possible that this is the truth, and Melissa has been set up as a red herring all along. If they are lies, Melissa is a very good actress, because she seems legitimately upset during the conversation.
Speaking of acting... As Spencer points out, Melissa's reaction to discovering Ian's body was not the reaction of someone who knew what they would find. If Melissa is "A," she deserves fifty Oscars.
That time she was in the hospital: Remember that time "A" seemed to save Spencer's life while Ian was attacking her, and then moved Ian's body? Remember how Melissa was in the hospital, and someone probably would have noticed if she somehow left and high-tailed it over to the church? Yeah. (Of course it's possible the mysterious savior was someone else, but that doesn't seem very likely at this point.)
Also, she' pregnant: Unless Melissa is faking her pregnancy (in which case she'd have to bribe a lot of doctors!), it's hard to see how she could accomplish all the sneaking around "A" does. Unless she outsources all of the physical work to a helper...
Of course, none of this is definitive, it also doesn't exonerate her from possible involvement in Ali's death, and there's a lot of evidence pointing in the other direction. But that's what we love about this show: It keeps us guessing!