Exactly, and the real risk is if you fail. Brakes can overpower motors on these cars. Even if you can exploit the system well enough to trigger full power (you

Exactly, and the real risk is if you fail. Brakes can overpower motors on these cars. Even if you can exploit the system well enough to trigger full power (you can't) you'd then have a high risk that the person would notice, hit the brakes, and stop in time. If they didn't stop in time, you'd have the risk that the accident wouldn't be fatal. This can easily lead to an investigation and a failed assassination. The lowest risk option is the one that works every time, despite how fun the movie plot scenarios are to think about.