...yes once you perceive emptiness the feedback loop stops. But to reach that point one needs to follow morality.
But being "100% ethical" is not a prerequisite for Nirvana. Its the realization of emptiness, the ability to see through delusions and unbind them, that is the prerequisite, that is Nirvana.
The state of being 100% ethical is impossible until one sees through delusions and gain understanding of emptiness. So our attempts to be ethical is just a means of training.
Following the precepts is just an exercise for the mind to strengthen it as it goes through the path to gain the knowledge of the Unbinding.
I guess to answer my original question "Can the knowledge of the Unbinding/realization of Emptiness/attainment of Nirvana be used for evil?",
It is possible theoretically I think but it will be unimaginable and will not make sense for a liberated person to do so.