In theory, yes. In practice, this is going to be very hard, because you need to create the software that turns the car into psycho killing machines. It is very unlikely that once you have hacked a car you could just sen the command "enable psycho killer mode" and it will behave as such. Instead you would have to teach the car how to behave on a murderous rampage, how to target fleeing pedestrians. To do that you would have to interact with all its sensors and disable all safety mechanisms, some of which may be implemented in hardware so that they cannot be touched by an over the air update. It would be possible to do that, but it would require a significant amount of effort for development and testing. Someone on the terrorist watch list building a test track, where he has cars trying to run over cardboard figures with images of pedestrians on them, would draw serious attention from law enforcement and other agencies, don't you think?