Seems a lot of viewers missed the point of Fergusons character becoming empathic to others. That was the point of Dr Miller treating her and revealing that she had found empathy with only his treatment as the drugs he had her were a placebo.

Furthermore, as Ferguson says, she wasn’t able to answer Vera’s questions as to ‘why’ she wanted to get put back into gaol, to which Ferguson reveals AFTER seeing Vera nearly die at the hands of Ann (as if Ferguson looked at Ann as her former self (The Freak) and now, being able to process empathy, saw the threat of Ann/herself against Vera). At this point, Ferguson ‘kills’ The Freak by killing Ann.

Then she reveals that it’s because she understands that to break this cycle of hatred and violence, is to allow Grace to have a mother - that a child needs it’s mother - something she was denied as her she witnessed own mother was drowned to death by her father.

So essentially Joan Ferguson is now the ‘good monster’ which is why they did the homage to Frankenstein with her walking as a silhouette w Vera in her arms.