Killing the brakes on a car would require mechanical interference/sabotage, not just software hacking. Aside from the exceptions snuxoll mentioned, I can think

Killing the brakes on a car would require mechanical interference/sabotage, not just software hacking. Aside from the exceptions snuxoll mentioned, I can think of two ways the brakes could be hijacked on a car with traditional hydraulic brakes in order to cause a crash. The first is abusing the antilock brake system to render the brakes less effective. Instead of reducing braking power when the wheels lock, it could be programmed to reduce braking power as much as possible in the event of panic braking at high speed. The second is to abuse the stability control system now mandated by law to be included on new cars in the US and many other jurisdictions to induce, rather than prevent/correct a skid. Stability control works by selectively applying the brakes on different wheels. Applying one front brake at a moderate level (not enough to lock the wheel and leave rubber on the road as evidence) would look just like an erroneous steering input to anyone investigating a crash as long as the tampering couldn't be detected electronically. For extra evil, use GPS or have an observer trigger it manually when the car reaches a location in which a sudden turn would result in an especially bad crash.