Gatsby admits to Nick that Daisy was driving the car and was the one who ran over Myrtle:
"Was Daisy driving?"
"Yes," he said after a moment, "but of course I’ll say I was. You see, when we left New York she was very nervous and she thought it would...
steady her to drive—and this woman rushed out at us just as we were passing a car coming the other way. It all happened in a minute but it seemed to me that she wanted to speak to us, thought we were somebody she knew."
Gatsby is a good person in his attempt to shield Daisy from blame for the accident and take the fall for her—a fall that will lead to his death.
On the other hand, Gatsby enabled Daisy to do what she and Tom always do, which is to get away with their misdeeds without accountability. Gatsby might have done her a favor in the end if he forced her to face that she killed a person. We might wonder why, as Gatsby describes it, Daisy lost "her nerve" and
turned away from the woman toward the other car, and then she lost her nerve and turned back. The second my hand reached the wheel I felt the shock—it must have killed her instantly
Daisy was willing to make a decision to hit another human being rather than another car, presumably because she thought hitting the other car would cause more damage and more trouble for her. She preferred to run someone over to getting hurt herself or even to having to stop and get out of the car. We also have to wonder if it occurred to her all of a sudden that this was Tom's girlfriend, and she saw a chance to get rid of a rival.
Whatever the reason, Daisy had a choice and made the one that cost another person her life—and was willing to let Gatsby take the blame for it. If she had had to take the responsibility for it, maybe she would have learned to be more careful about wrecking other people's lives in the future, knowing there were consequences for her. On the other hand, Gatsby showed that his love for Daisy was pure to the end.