sadly id expect you would be up on a manslaughter charge if they died.

You cant legally set up traps to hurt burglars in the home or on your property so i suspect that would be the same on a car. Probably more so as a car on the public road is considered a public place in law (hence weapons in cars are illegal).

In this country even putting laxatives in your food to catch out student house fridge thieves or lunch thieves at work is considered illegal.

The law is strange. it views the same event as seperate crimes dispite clearly being closely linked/related.

eg in food example above, although someone shouldnt steal your food, that is a seperate legal issue to you poisoning them regardless of the fact that they wouldnt be poisoned if they didnt commit a crime.

Same with burglars getting hurt in your home. Despite them breaking the law by being there, it is entirely seperate to the "health and safety" breach that caused thier injury. madness really. H&S is about preventing avoidable accidents and taking "reasonable" precautions to reduce risk. Insane that the law considers it "reasonable" to take precautions to avoid hurting burglars. Insane how its considered "reasonable" that a burglar is on your premises.......

So fitting a device that disables a major safety feature like brakes would almost certainly be illegal. Im sure the police and the CPS would also go down the "risk to the public" route in thier prosecution as the car with no brakes could mow down a bus stop of kids or similar. Again it would be your fault despite the fact the criminal scum shouldnt be in your car, let alone driving it.