Gwyneth's being the killer made sense, and it tied back to when she and Trisha first came on the show. Their relationship was always strained..and remember she would interfere in her kids lives.
I think Trisha's 'death' started her mental breakdown. She had a fling with Buck, miscarried, was lost and listless. I recall her trying new jobs like bartending in order to find purpose. She also was motherly to Janie and later Steffy.
The final straw was Trisha being found alive and rejecting her. I believe that happened in spring 1995. And the killings started a bit afterwards.