I think Darlie and Diane had the same motive, and it’s a very common one. It doesn’t always end up in a mother (or father) murdering their child, but it often does end in murder or other destruction of the child. The parent wants to move on in life. Sometimes (many times) it’s a boyfriend who does the killing.
With Darlie and Diane, they did it themselves.
With regard to Darlie’s “injuries”, they were actually very minimal. I do think that Downs was very intelligent, Darlie not so much. In trying to make herself appear to be a victim, she actually got very close to accidentally doing something serious to herself!
At any rate, aside from the imperfect investigation (and they’re always imperfect) I have not a shadow of a doubt that Darlie did it. Glad she’s “off the streets”, but very unjust that one who chooses to end innocent life gets to go on living hers.