The problem is that it looks like the pilot deliberately deactivated any systems that broadcast the jet’s position. Short of an independent (solar/battery or ram air turbine powered) system which is totally independent of the plane’s electrical system, all systems on an aircraft need to be on a fuse or breaker for safety reasons.
Things need to be turned off if they cause wiring to overheat or interfere with the function of other electrical systems. ADS-B is just a transponder that sends out GPS data. It will also be on a fuse or breaker, and thus subject to malicious deactivation.
In short, if someone determined to cause harm gets control of an aircraft, there’s damn little anyone can do. Even if the plane did broadcast its location, it wouldn’t have helped anyone aboard if at least one of the pilots was bent on murder and suicide.
