Right, yeah, I was forgetting the details of the plan. But the point is the same: despite Dexter's belief that he can stab his problems away, killing LaGuerta doesn't really improve the situation. Regardless of how convincingly the scene is staged (and "Dexter is a forensic expert" is a cheap plot device to keep relying on), there's no way LaGuerta and Estrada dying within days of her arrest of Dexter doesn't make someone
immediately suspicious... unless the writers ignore basic dramatic logic to produce their desired outcome. To an extent they already are, and this interview
with the showrunner suggests they may continue to, at least in the short term. And they probably have to, to stretch things out over another twelve episodes. Which is part of why telling this story over two seasons was probably not the best idea.