That cop is morally responsible for her death.
He instigated the stop - he intentionally tailgated her, at speed, forcing her to change lanes, and then pulled her over (almost certainly because she was out-of-state) - and then he provoked her into giving him any pretext whatsoever to arrest her - he already knew she was irritated, he goaded her about that, and then he gave her an order that he knew she would disobey - to put out her cigarette - and when she refused, he ordered her out of her car.
Let's be clear about that: she did not voluntarily exit her car, she got out because she was ordered out. And that order was the start of arrest - once the cop orders you to do something like get out of your car, and then pulls the door open when you refuse, you are in custody. You are most definitely not free to leave.
That was a clear example of the daily abuse of power cops engage in all the time. And that is why no cop can be trusted, period. There was no way for this poor woman to know that she was dealing with an animal, an evil-doer, because he looked just like every other cop; the only way to minimize the risk of something similar happening to you is to simply distrust every cop you deal with, have as little to do with them as possible, and shun them the same way you would a leashed pit bull.
Apparently it is now a capital offense to be Smoking-While-Black in Texas.