Song Yet Sung discussion

What the heck?! Why did Liz kill the Woolman? I'm completely confused by this and likewise confused that no one in the reviews above seems to be talking about it.

The only reason I can think of is to be able to reach his son so the boy would be free to become an ancestor of MLK. But her dreams had already foreseen this would happen--nothing to suggest the boy must come out of the swamp for it to occur or that Denwood would help release him from jail rather than the Woolman somehow saving him.

So why did she do it???