Why did Nicola Walker leave Unforgotten; why did Cassie Stuart die?

Why did Nicola Walker leave Unforgotten; why did Cassie Stuart die?
Why did Nicola Walker leave Unforgotten after the season 4 finale?
Did her character Cassie Stuart have to die?
If you are asking these questions in the wake of tonight’s finale on PBS, we more than understand.
Losing the character at this point has to be one of the more sad and traumatic TV deaths in recent history — it’s not something that we wanted, and it’s absolutely something that will take some time in order to digest.
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We do know already that there is going to be a season 5 coming; in that, you will see how the character of Sunny tries to move forward.
We hope that there is at least some effort paid to remember this character — she was, after all, such an integral part of making this series what it was for so many years.
Let’s get back now to the matter of  why  Walker is no longer a part of the story.