Speaking as a passenger, I'm more reassured by the two-person cockpit idea than the professionals on here seem to be.
I think most situations where a single individual is given private, unmonitored power over the lives of other humans is inherently dangerous. There will always be a small proportion of people for whom the temptation to abuse that power may become irresistible at certain times - look at doctors, nurses, and the police. Almost all of the biggest scandals happened because the person concerned was handed a high degree of trust on account of their profession, and could operate in privacy.
Another human being in the cockpit does not guarantee that a rogue pilot won't smilingly put the plane into a calculated descent. However, it reduces the opportunity. The rogue pilot is likely to have to make conversation with the crew member - thus reducing the time to do something naughty - or will have some concern that the CC has flying skills and can see the naughtiness being done, or will find it necessary to maintain the mask that covers the psychosis.
Of course, the presence of CC does not guarantee that the rogue pilot won't smash his colleague's head with an iPad and do bad things. But it certainly doesn't increase the chances, and that's the best that can be hoped for.
Edited to add: I see I'm in agreement with fellow passenger ZA9RA22.