D lives in a Model Penal Code jurisdiction where general principles of criminal law fill the gaps in the statutes. D is driving down a road that's not a highway but allows people to go relatively fast (maybe 40 mph). His foot is on the gas pedal and his ankle is flexed, so his calf muscle is engaged to press the pedal. Intent on killing himself, V jumps on the road in front of D's car. D does not shift his foot off the gas, and D hits and kills V. There happens to have been a camera with a view, and the video and accident forensics prove that D could have hit the brake and stopped before reaching V, or swerved away from V. D claims he just froze with panic and couldn't think through what to do. The DA's office thinks that proving mens rea and getting around the intervening actor doctrine may be tough, but wants to go for it. D is charged with negligent homicide. Is D's actus reus an act, holding the gas pedal down, or an omission, the omission to brake or swerve?
Arguably, between the two options presented, the actus reus needs be an omission of breaking or swerving, simply because merely taking one's one foot off the accelerator would have D rolling and hitting anything in a straight line for probably thousands of feet (Rolling friction alone on tires specifically designed to reduce said friction will take quite a while bleed off the momentum of an two tons of car (average car weight) going 40mph; as an anecdote, my car lost power on a highway going ~30 mph, and coasted more than a quarter of a mile before it came to a complete halt). In which case, D is failing to discharge the duty they accrued when their entrance onto the street at speed transformed a safe, if potentially highly volatile, area (an empty street) into a perilous one.
Alternatively, one could argue that it is an action of "driving in an unsafe manner, which resulted in a fatality".
Per the facts presented: If V "jumps out at D's car in order to kill themselves", D is driving at the legal speed limit (40 mph), and the camera analysis proves that D could have stopped in time, then V must be in excess of 140 feet away (per my old CA DMV booklet from when I got my license, expected breaking distance is 80 feet and distance to perceive and react an occurance is another 60 feet; A quick google search suggests that swerving would take 60 feet, for a total of 120 feet to swerve). If it is forensically "proven" that D could have stopped in time, but didn't, it would have required this "panic state" to have lasted several seconds (40 mph is ~58 feet/second), in which case this "vehicle" would have arguably been out of control for hundreds of feet in a populated area, which is inherently unsafe.
Note that the Model Penal Code defines negligence thusly:
"A person acts negligently with respect to a material element of an offense when he should be aware of a substantial and unjustifiable risk that the material element exists or will result from his conduct. The risk must be of such a nature and degree that the actor's failure to perceive it, considering the nature and purpose of his conduct and the circumstances known to him, involves a gross deviation from the standard of care that a reasonable person would observe in the actor's situation."
The prosecution would argue that not breaking creates a risk of injury to V, and that the "reasonable person" would break; D (or their lawyer) could assert that their panic state rendered them unable to break; I believe that this defense (inability) would be an affirmative defense that the defense would need to bear the burden of proof for.
Ultimately, I agree with DaleM: the actus reus is hitting V with the car. Legally it doesn't matter whether or not you take your foot of the accelerator, break or swerve on an empty road, so long as you don't hit anyone or anything (except for maybe it might be grounds to check your sobriety, etc.).
2B Causing death by careless, or inconsiderate, driving:
A person who causes the death of another person by driving a mechanically propelled vehicle on a road or other public place without due care and attention, or without reasonable consideration for other persons using the road or place, is guilty of an offence.
s.3ZA defines careless driving:
(2) A person is to be regarded as driving without due care and attention if (and only if) the way he drives falls below what would be expected of a competent and careful driver.
(3) In determining for the purposes of subsection (2) above what would be expected of a careful and competent driver in a particular case, regard shall be had not only to the circumstances of which he could be expected to be aware but also to any circumstances shown to have been within the knowledge of the accused.
Here, an assessment of the actus reus is not specifically focussed on whether the accelerator was, or was not, depressed but rather it takes holistic view of whether the driving fell below the standard expected by a competent and careful driver.
In the end, it will be a matter for the jury after considering the evidence:
...accident forensics prove that D could have hit the brake and stopped before reaching V, or swerved away from V. D claims he just froze with panic and couldn't think through what to do...