Do airliners have an "anti-pilot" warning system?
The system you're asking about is called radar: ATC should monitor it to see what the plane is doing and ask what is going on when something unexpected happens.

It's hard to "automate" these kinds of alerts. For example, a sudden dive can also be cause by an uncontrolled control-surface hard-over (like the boeing 737 rudder problems that brought down 2 planes), or a loss of cabin pressurization (which requires a rapid descent to an altitude where your passengers can breathe).

The other option is having a second person in the cockpit who can wrest control away from the disobedient pilot (or let people in the cockpit to help).