On the day of Jessica's death, Peter Hastings had asked her if they still had an "agreement" about Spencer, but on having just discovered that Alison was alive, Jessica didn't give him a clear answer. If Jessica had suspected that Spencer had killed Bethany Young, would Peter have killed her to ensure her silence?
We all know the extremes Melissa will go to, to protect her sister. If she also knew that Jessica suspected Spencer, could she have killed her? Jessica's cause of death was taking the wrong medication, and we later found out that the medication she had actually taken belonged to Peter Hastings. Melissa could have easily taken it.
The fact that Jessica was found with Peter Hastings' pills in her system could also mean that someone was trying to frame him or point the blame away from themselves. If this is the case, the killer would probably be someone outside of the Hastings family. Sara knew how close Charlotte was to her mom, could Sara have wanted to punish Charlotte for all she had put her through?
How many reasons does Mary Drake have to hate her sister? Jessica let her take the blame for a crime she did not commit, Mary Drake was then sent to Radley as a consequence, and after all that, Jessica THEN took her child away from her! Could Mary Drake have killed her sister as revenge for all the hurt she had caused her?