The price for one life was another life.
If they killed Caroline and Liberty, they would likely be caught within days. Then, they would face legal punishment, possibly the death penalty. What would be the point of that?
What they wanted was control over the Fisher family, but if they were sentenced to death, the Fisher family assets would only go to others.
“Adrian, a life for a life. If we kill someone, we won’t be able to escape—what’s the point of that? What if our plan fails, and inste