The decision whether or not to prosecute the officer was up to the district attorney in King County. The problem exists nationwide in these circumstances because you are asking a prosecutor that works with the police in every single case that comes across their desk to make a decision whether or not to pursue charges against a cop, that they may need help from in the future. Special prosecutors should be used in these situations in my opinion.

Margaret Mayer was actually convicted of failing to stop and render aide in a deadly traffic incident. Under this law, there is no evaluation over whether or not the person is at fault for the accident or not.

TX TRANSPORTATION CODE 550.021
The operator of a vehicle involved in an accident that results or is reasonably likely to result in injury to or death of a person shall:
(1) immediately stop the vehicle at the scene of the accident or as close to the scene as possible;
(2) immediately return to the scene of the accident if the vehicle is not stopped at the scene of the accident;
(3) immediately determine whether a person is involved in the accident, and if a person is involved in the accident, whether that person requires aid; and
(4) remain at the scene of the accident until the operator complies with the requirements of Section 550.023.
… …
(c) A person commits an offense if the person does not stop or does not comply with the requirements of this section. An offense under this section:
(1) involving an accident resulting in:
(A) death of a person is a felony of the second degree

A second degree felony is punishable by 2 to 20 years in prison. You are right, but her motivations, fears, etc. are not considered under Texas law for prosecution in this situation. If you are in an accident you know could involve serious injury or death and you do not stop, you can be found guilty.

Tristan, would you comment on the Margaret Mayer case, the lady who was convicted in Houston of hit-and-run of the bicyclist. What they keep NOT talking about is that the bicyclist was riding at night in a lane of traffic, wearing dark clothing and NO lights on her bike.

I think frightened people leave the scene of an accident because they know they will be crucified for accidentally hitting someone they could not even see in the dark!