I didn't catch the TV news on this today though I'm sure it will be on later. I read on Cincinnati.com News Flash that, according to prosecutors, Kara said she intended to run off with David. He said the same thing. There was no mention of her involvement or beforehand knowledge of the shootings.
"...just being used though."
When I consider the hormonal effects alone which teens have to navigate past on their way to adulthood, maybe this fellow was obsessed with Kara, couldn't imagine being without her.
What we are doing is speculating because we want to figure it all out and we just don't have all the facts.