Do airliners have an "anti-pilot" warning system? In February 2014 a co-pilot hijacked Ethopian Airlines flight 702 and took it to Switzerland.

Now in March there is some speculation that Malaysia Airlines flight 370 may have been hijacked and destroyed by the pilots - maybe they took a nose dive into the Andaman Sea?

So my question is this: is there an automatic or say anti-pilot warning system on commercial airliners?  

In other words, a system that is non-maskable (can't be disabled by the pilot) and which will automatically warn ATC about unexpected conditions (like a sudden decrease in altitude)?
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).