The almost-love-triangle between Philip, Elizabeth, and Martha was never going to end well. When Elizabeth showed up to fetch Martha in season 4, episode 7, I was certain she would kill her and was shocked when she didn't. I think that rare act of restraint was out of love for Philip. Elizabeth knows how much her spy husband cares about his fake wife, so she spared Martha's life. If she had killed Martha, would it have been driven by jealousy, or again by her love for Philip and his protection?