This is overcomplicating things. If I pilot wants to be a mass murderer there is not much that can stop them. A brisk push whilst low, roll it over and pull the wings off when the other bod is in the loo, unfortunately there is nothing other than getting rid of the pilots that can stop this, but that will open up a whole other can of worms.
There needs to be either better mental state monitoring and support, or severe consequences. If proven that one of the pilots did this their family should be held accountable and financially liable, it would destroy their future and make others think twice about this despicable road. Some may think it unfair, it was unfair to the thousands of dead and grieving this sort of tragedy effects. The family know the perpetrator better than anyone else, there should be non punitive whistleblowing to get the person support.